Hi Joachim,
The image comparison fails due to font smoothing in the IE 9 screenshot (on the right). It's a
known problem with Internet Explorer 9, which always uses font smoothing regardless of Windows settings and doesn't have an option to turn it off.
A possible solution is to roll back to IE 8, which supports turning off font smoothing. There's also a
hacky workaround, which involves registry tweaking and placing two extra DLLs in the IE 9 folder.
If neither is an option, I'm afraid, you'll have to resort to using pixel tolerance, as explained in
Ignoring a slight difference between images.