It's such a small detail that I wouldn't have otherwise noticed (because I don't build animations, so it doesn't cross my mind), but now that I know about it, I think about it all the time.
I think there was also an article about it on The Old New Thing, but I can't seem to find it anymore.
(These days I would have included more comments.)
Naturally, there was a fallback to pure CSS menus, because we weren't barbarians.
I like predictable, consistent, and fast.