I have a Bitmap Check (I don't like them, but no other way, to show required files on our website it puts a Red Squiggly under the required fields)
But when I run this script on our testing box, the fonts on the screen look just a little different. (almost a little wider) the size of the fonts are the same.
Both computers have IE8 and are running Windows xp.
They work one machine but not the other.
You may want to ensure the ClearType and other font settings match.
But if the PCs aren't identical, you can have bitmap differences. This goes beyond color and resolution settings, it can be different video drivers and/or other low-level hardware/software discrepancies.
If the difference is small, you can use the Criteria tab to set a # of pixel differences threshold - but from your description I doubt this will help.