I would concur with Magick. I doubt it is a bug in IM I wrote (or re-wrote) most of the animation code to make sure it works correctly.
Also I would not use a value like '2' for a dispose method. What do you think '2' is?
Looking things up '2' is equivelent to 'Background' Disposal. Something that a lot of old browsers (for example Mosaic, if you were around before the 'Super-Highway') did not implement properly.
Actually NO browser implements the GIF specification properly as ALL of them actually ignores the 'background' color setting. That is because in a browser, background is really what is behind the animation, (that is 'transparency') and NOT the GIF background meta-data setting. So no one uses it, not even IM.
Also have you compared the verbose output of the ImageReady version with the IM version, it may be that IE does not handle a specific dispose method, and ImageReady avoids that method.
A good way of finding out all the animation settings being used by a GIF animation is to use the script... "gif2anim"
http://www.imagemagick.org/Usage/scripts/gif2anim
See the introduction in
http://www.imagemagick.org/Usage/anim_basics/#sequence
This reports all the IM commands that would have been used to originally build the animation, as well as seperate the animations frames AS IS, with any and all their optimizations.
Actually I recommend you read that whole section
http://www.imagemagick.org/Usage/anim_basics/
In which I tried to completely explain all aspects of GIF animations
the next section on Optimization is also a recommendation so as to understand
why a working animation can look so complex and convoluted when viewed as raw data.
http://www.imagemagick.org/Usage/anim_opt/