Width of em is larger than width of "m" or "M"

The width of an em as in text-indent: 5em is much larger than the width 
of an "M" or an "m".  In this case, 5ems is about double the width of 5 
"M"'s.  I am using Amaya GL 6.4 on Windows XP Home Edition.  My display 
settings are 85 dpi, 1024x768, 32bit.

-- 
Brant Langer Gurganus
http://troop545.cjb.net/brant.htm