You're assuming it's intentional. The first thing that comes to my mind is inconsistent state between multiple authentication servers.

Normally it would to be prevent users from reusing their most recent N passwords, for security. I don't believe Apple does that anywhere, though.

I suppose thinking about it, as long as they're hashed and salted, it's less of a problem... but it's still crazy that the system could fail that way.

I can confirm that they do.