Units of measurements

I'm a little concerned about the way the em and
ex units used in style sheets are implemented
in IE4.

I chose to use a monospaced font and specified
a left margin of 1 em unit and found the margin
did not match the width of the character 'm'.
Rather it turned out about 1.5 times larger than
the 'm' character.  Also, the ex unit turned out
to be slightly less than the width of the 'x'
character.

Because of this, I am unable to specify a
margin that is a multiple of the width of the
characters in a monospaced font.  Can
someone explain why this is so?  The
problem also exists in NN4.