I just noticed. They are opposite of what I though they would be!!
Have a look: https://superuser.com/users/8829/eskay
I suspect it's just a sort ordered by (number of posts, name) that's descending, instead of ordering by number of posts descending and name ascending.
Given that this goes against the expected ordering, I think I'd call this a very minor bug. Mind you, the tags page itself doesn't seem to have any (obvious) secondary ordering...