Yeah, I've wondered about that. They definitely have different part numbers, and one is even painted differently; and they don't cost exactly the same. But I haven't seen any noticeable other visual or functional diference.
I wonder if one is actually a bit longer on the internal threaded shaft or something, and is actually screwed out a few more threads when installed. Never checked final installed length to compare.

We can file that one with the other mystery of life - the EHA module.

DG