Problem with parallel stream - it parallelizes per batch not per item

ParallelStreamProblem.java

import java.time.Duration;
import java.time.Instant;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

/**
 * @author Tomasz Linkowski
 */
public class ParallelStreamProblem {

  public static void main(String[] args) {
    new ParallelStreamProblem().processItems();
  }

  private final Instant started = Instant.now();

  private void processItems() {
    Stream.of(Batch.values())
            .flatMap(Batch::itemStream)
            .parallel()
            .forEach(this::processItem);
  }

  private void processItem(String item) {
    long elapsedSeconds = Duration.between(started, Instant.now()).getSeconds();
    System.out.format("[%d] Processing %s on %s%n", elapsedSeconds, item, Thread.currentThread().getName());
    try {
      Thread.sleep(1000);
    } catch (InterruptedException ex) {
      throw new RuntimeException(ex);
    }
  }

  private enum Batch {
    A(3),
    B(10);

    private final List<String> items;

    Batch(int itemCount) {
      items = IntStream.range(0, itemCount).mapToObj(i -> name() + i).collect(Collectors.toList());
    }

    public Stream<String> itemStream() {
      return items.stream();
    }
  }
}

result.txt

[0] Processing B0 on main
[0] Processing A0 on ForkJoinPool.commonPool-worker-1
[1] Processing B1 on main
[1] Processing A1 on ForkJoinPool.commonPool-worker-1
[2] Processing B2 on main
[2] Processing A2 on ForkJoinPool.commonPool-worker-1
[3] Processing B3 on main
[4] Processing B4 on main
[5] Processing B5 on main
[6] Processing B6 on main
[7] Processing B7 on main
[8] Processing B8 on main
[9] Processing B9 on main