Is there a discrepancy in the amount of registered users?

Question

Just last week we were celebrating hitting 10k users and the past few days we went back down below 10k and we're just now approaching it again.

Is there some regular culling of accounts or something?

Answer 1

Yes, there is auto-culling, primarily of unregistered (cookie-based) accounts.

See

http://blog.stackoverflow.com/2009/02/when-is-an-account-abandoned/