"absolute" vertical position seems awfully relative

[Dave Kemper]

Here is what the groff Texinfo manual says about prepending a vertical
bar to the number associated with a movement request or escape:

"Similarly, a leading '|' operator indicates an absolute position.
For vertical movements, it specifies the distance from the top of the
page..."

This seems obvious and straightforward, until you try it out.

Here is my sample input file.  It uses hard-coded basic units suitable
for PostScript or PDF output.

\v'|30000u'At 30,000 units.
.bp
\v'|30000u'At 30,000 units.

The '|30000u' argument to the \v escape specifying an absolute
position, the text should appear in the same place on both pages.
This is what you get when using basic groff.

But specify a macro package of -ms, -me, or -mm on the command line,
and the results change: the text is now in noticeably different
positions on the two pages.  The placement with all three macro
packages is remarkably similar.

Why should a macro package change groff's concept of an absolute
distance from the top of a page?

At first blush this seems like a bug.  But Heirloom troff does the
same thing, so maybe the so-called absolute position is relative in
some obscure but intended way?