clipping w/stretchdraw & metafile ???

I think I've missed something  fundamental about using metafiles in
conjunction with stretchdraw.  I had assumed that the metafile graphic
would be wholly drawn within the trect passed as a parameter... but
that, apparently, is not always the case.

I found that a support application I had just bashed out produced
differing results when run on different screen resolutions.  The app
translates HPGL graphic instructions, painting on a metafile canvas.
The metafile is used via stretchdraw to paint a canvas on the form
and/or to a printer.  When run from progressively lower resolution
desktops, the graphics produced by stretchdraw were increasingly too
large for the trect specified.

I found that I had to apply an adjustment to reduce the bounds of the
desired trect, based on the current screen height, using a different
adjustment for screen & printer. (i.e. specify a trect smaller than
the bounds of the canvas to be painted).

Can someone fill me in on what's happening in the background? What
prevents any given metafile from stretchdrawing within the bounds of
any given trect?

Nick Howell