ok maybe they are different. Im more concerned with look of grain. T-type grain (refined at smaller level) and classic grain (more groove), also ability to push ISO. I have used old apx400 and apx100 and find them indeed different as you said. They are hard to push over also.
VERY different indeed...APX400 is (was) ISO250 at best but the grain is superb in my opinion, especially with Rodinal.