columns displayed on one line in frame / window

[Mickey Ferguson]

I have emacs 21.3.1 running on XP SP3.  When I evaluate the _expression_ using M-: (Meta-colon),

 

    (frame-width)

 

it returns 72.  However, if I look at the number of columns I can display before the line wraps, it's actually 80.  Isn't frame-width supposed to be the same as the number of columns that can fit on one line?  (I'm using a fixed-width font, specifically

 

  (set-default-font "-raster-Terminal-bold-r-normal-normal-12-90-96-96-c-*-ms-oemlatin")

 

Likewise, if I set the frame width with

 

    (set-frame-width (selected-frame) 72)

 

it sets up the window so that 80 columns fit on one line.  What gives?  Or is there another function I should use to obtain the number of columns that fit on one line, and likewise, another function to set how many fit on one line?