Well, you discovered one of the HP curses that the ACAD programers wrote
in. When you plot directly, ACAD queries the plotter as to the origin. When
plotting to a file, HPGL calculates the vectors from the center of the
sheet.
The best way to keep from going insane is the use the same sheet size in
the ACAD menu as what you really have loaded in the plotter.
<<John, BOLDER Designs>>
PS. HPGL/2 uses the lower left corner as the orgin just like everyone else.