Create a grid of equal-sized cells from an array of geometries in PostGIS

create_fishnet.sql

/*
Author: Tom Caruso

Divide up a box into multiple equal-size cells.

Helpful to split up a coverage of geometries into a gridded

Usage:

SELECT create_grid(array_agg(geom), 10, 10, 4326) FROM my_geometry_table;

*/

DROP TYPE IF EXISTS grid_record;
CREATE TYPE grid_record AS (cell_id INT, geom GEOMETRY);

DROP FUNCTION IF EXISTS create_grid(box, int, int, int);
CREATE OR REPLACE FUNCTION create_grid(box box2d, cells_x int = 2, cells_y int = 2, srid int = null)
  RETURNS SETOF grid_record AS
$$
DECLARE
  box_geom GEOMETRY;
  geom GEOMETRY;
  cell_id INT;
  width FLOAT;
  height FLOAT;
  cell_x INT;
  cell_y INT;
  dx FLOAT;
  dy FLOAT;
  x_start FLOAT;
  y_start FLOAT;
  ret grid_record;

BEGIN
  box_geom := box2d(box)::GEOMETRY;

  width := st_xmax(box_geom) - st_xmin(box_geom);
  height := st_ymax(box_geom) - st_ymin(box_geom);

  dx := width / cells_x;
  dy := height / cells_y;

  cell_id := 1;

  FOR cell_y IN 1..cells_y LOOP
    y_start := st_ymax(box_geom) - (cell_y * dy);
    FOR cell_x IN 0..(cells_x - 1) LOOP
      x_start := st_xmin(box_geom) + (cell_x * dx);

      geom := st_makebox2d(
          st_makepoint(x_start, y_start),
          st_makepoint(x_start + dx, y_start + dy)
      )::GEOMETRY;
      IF srid IS NOT NULL THEN
        geom := st_setsrid(geom, srid);
      END IF;

      SELECT cell_id, geom INTO ret;
      cell_id := cell_id + 1;

      RETURN NEXT ret;

    END LOOP;
  END LOOP;
END;
$$ LANGUAGE plpgsql;

I believe that I find a behaviour not expect. When I create a fishnet from polygon the result of grid has exceeded from limit of polygon source. Do you believe that it is possible to create a fishnet from polygon with the same size of original?