Steve, you where hit by IE box model problem. The IE add up to 3 pixel for each box.
To prevent this you must change your style with Tanteks-Hack commands in your style. But this can be only done for one version because every IE version have other hacks. I found a german website wich describe the problem:
http://www.perun.net/2004/07/04/fehlverhalten-von-ie-im-boxmodell/
