What could cause uids to not be numbered consecutively?
So, it turns out this is indeed normal since Drupal 7: the users table is not auto-increment but uids are created with a sequences API that is shared with actions and batch jobs. Uids can therefore be non-consecutive.

Several reasons for changing back from auto-increment table in D6 are pointed out here by chx.

(From reading chx's response to this blog post)