HP 4 FAMILY - Manual (Page 62)

The example shown in Figure 2-1 illustrates the concepts
of the two models. Assume a rectangle extends from
coordinate position (1,1) to position (3,4). As shown below,
each model produces a different result. (Since PCL printers
print only at intersections, grid centered pixel placement is
implemented as shown on the right.)
The grid centered model produces a rectangle that is one
dot row thinner and one dot row shorter than the grid
intersection model. The grid intersection model is the PCL
default.
Note The grid centered method is used by Microsoft Windows.
Figure 2-1. Pixel Placement
2-26 Printer-Specific Differences