From what I can tell, the two browsers are the same but different. GE does not actually use IE. It uses the "INET subsystem." That is the layout engine and some related components. I suspect INET uses seperate cookie and cache storage for each client (which would include IE).

If so, just because you have some file (pick a big one if you test this with broadband) in one cache, do not expect it to be in the other.