trying to understand the difference between SF2 own session system and PHP default one

RecordExporterController.php

<?php

namespace Imbc\RecordBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

class RecordExporterController extends Controller
{
    public function indexAction( Request $request )
    {
        // TODO Change this to use the default PHP session mechanism
        // in order to check if symfony's own could be the culprit
        if( !isset( $_SESSION['dataset'] ))
        {
            $oldData = array();
        }
        else
        {
            $oldData = $_SESSION['dataset'];
        }

        ////////////////////////////////////////////////////////////////////////
        //
        // Symfony2 own session system
        //
        ////////////////////////////////////////////////////////////////////////
//        $session = $request->getSession();
//        $oldData = $session->get( 'dataset' );
//        $session->set( 'dataset', NULL );
        $form = $this->getForm();
        $criteria = array();
        $data = array();
        if( $request->isMethod( 'post' ))
        {
            $form->bind( $request );
            if ( $form->isValid() )
            {
                $criteria = array(
                    'exported'  => $form['exported']->getData(),
                    'from'      => $form['from']->getData(),
                    'to'        => $form['to']->getData(),
                    'view'      => 0 === $form['view']->getData() ? FALSE : TRUE,
                    'type'      => $form['type']->getData(),
                );
                $data = $this->getDoctrine()->getManager()
                            ->getRepository( 'ImbcRecordBundle:Record' )
                            ->getExportData( $criteria );
                if( $data != $oldData )
                {
                    unset( $_SESSION['dataset'] );
                    $_SESSION['dataset'] = $data;
                }
                $_SESSION['criteria'] = $criteria;
        ////////////////////////////////////////////////////////////////////////
        //
        // Symfony2 own session system
        //
        ////////////////////////////////////////////////////////////////////////
//                if( $data != $oldData )
//                {
//                    $session->set( 'dataset', NULL );
//                    $session->set( 'dataset', $data );
//                }
//                $session->set( 'criteria', $criteria );
            }
        }
        $routes = $this->get( 'router' )->getRouteCollection();
        $dataset = $this->diffSubArray(
                isset( $_SESSION['dataset'] ) ? $_SESSION['dataset'] : array() ,
                $data
        );
        $remainer = abs( count( $dataset['new'] ) - count( $dataset['old'] ));
        ////////////////////////////////////////////////////////////////////////
        //
        // Symfony2 own session system
        //
        ////////////////////////////////////////////////////////////////////////
//        $dataset = $this->compareDataset( $session->get( 'dataset' ), $data );
//        $remainer = abs( count( $dataset['new'] ) - count( $dataset['old'] ));
        return $this->render( 'ImbcRecordBundle:Exporter:index.html.twig', array(
            'form'                  => $form->createView(),
            'data'                  => $data,
            'sessionData'           => $_SESSION['dataset'],
            'criteria'              => $criteria,
            'routePatternUpdate'    => $routes->get( 'record_exporter_update' )->getPattern(),
            'routePatternExport'    => $routes->get( 'record_exporter_excel' )->getPattern(),
            'somethingUnique'       => time(),
            'remainer'              => $remainer
        ));
    }

    public function getForm()
    {
        return $this->createFormBuilder()
                    ->add( 'exported', 'checkbox', array(
                        'required' => false,
                    ))
                    ->add( 'from', 'date', array(
                        'label' => 'From (Date)',
                        'attr' => array( 'class' => 'datepicker' ),
                        'widget' => 'single_text',
                        'format' => 'yyyy-MM-dd',
                        'input' => 'datetime',
                    ))
                    ->add( 'to', 'date', array(
                        'label' => 'To (Date)',
                        'attr' => array( 'class' => 'datepicker' ),
                        'widget' => 'single_text',
                        'format' => 'yyyy-MM-dd',
                        'input' => 'datetime',
                    ))
                    ->add( 'view', 'choice', array(
                        'choices' => array(
                             'Non-Exported',
                             'All Records'
                         ),
                     ))
                     ->add( 'type', 'choice', array(
                         'choices' => array(
                             'Alpha Records',
                             'Beta Records',
                             'Epsylon Records',
                         ),
                     ))
                     ->getForm();
    }

    public function setExported( $id, $exported )
    {
        $em = $this->getDoctrine()->getManager();
        $recordRepo = $em->getRepository( 'ImbcRecordBundle:Record' );
        if( NULL !== $record = $recordRepo->find( $id ))
        {
            $record->setExported( $exported );
            $em->persist( $record );
            $em->flush();
        }
        else
        {
            throw $this->createNotFoundException( 'No record has been found for id: #' . $id );
        }
    }

    public function exportAction( Request $request )
    {
        if( NULL !== $data = $_SESSION['dataset'] )
        ////////////////////////////////////////////////////////////////////////
        //
        // Symfony2 own session system
        //
        ////////////////////////////////////////////////////////////////////////
//        $session = $request->getSession();
//        if( NULL !== $data = $session->get( 'dataset' ))
        {
            foreach( $data as $row )
            {
                if( 'true' === $request->get( 'exported' ) )
                {
                    $this->setExported( $row['id'], TRUE );
                }
            }
            return $this->get( 'excel_export' )->export( $data );
        }
        return new Response();
    }

    private function diffSubArray( $left, $right )
    {
        $newArray = array();
        $oldArray = array();
        if( count( $left ) > 0 )
        {
            foreach( $left as $element )
            {
                if( !in_array( $element, $right ))
                {
                    $newArray[] = $element;
                }
            }
        }
        if( count( $right ) > 0 )
        {
            foreach( $right as $element )
            {
                if( !in_array( $element, $left ))
                {
                    $oldArray[] = $element;
                }
            }
        }

        return array( 'new' => $newArray, 'old' => $oldArray );
    }
}