[Groff] Math symbol upside down?

[Clarke Echols]

I am playing around with a little project and decided to make a
drawing using groff accompanied by mathematical analysis text.

I decided to use the \(~= sequence for the "approximately equal"
symbol in the math table, using the eroff manual I have from the
late 1980s as a reference.  In that manual, the symbol is a
horizontal bar with a tilde bar above it, but in the groff package
I got with cygwin, the tilde is on the bottom.  My PostScript
tutorial by Ross Smith indicates that the math font character is
actually a pair of tildes.

What's going on?  I've never seen a tilde on the bottom before.
Is this a take-off spoof like GNU's not Unix? :-)

Clarke