nroff mdoc character errors

[Warren Block]

man mergemaster | hd | less -p'80 90'

shows 0x80 and 0x90 characters where lines should be hyphenated. 0x80 
0x90 shows as "-" in an xterm, but as three non-English characters on a 
real console.  Likewise, parens come out as 0xa8 and 0xe2.

groff works correctly:
gzcat /usr/share/man/man8/mergemaster.8.gz | groff -Tascii -mdoc -

nroff does not:
gzcat /usr/share/man/man8/mergemaster.8.gz | nroff -mdoc -