... doh, was creating all users with the same ID. Which raises some serious questions as to why the tests were passing before...
Also wasn't deleting users when deleting their events
Aha, a reproduction of the issue at last. Deleting data for a user while an add request is also in the queue results in the deletion being processed before the addition, which results in the user reappearing.

The count is never wrong, I think, but a deleted user can reappear in the results.