What could cause uids to not be numbered consecutively? I just noticed something weird in a site I'm working on. The first account is user/1 as usual. A second account I created two weeks later is user/20, and a third one a couple days later is user/32. Etc. Two accounts I just created right after each other are numbered consecutively as expected.

There are no custom modules modifying the registration process or anything. I wonder what could cause this, and if there's something I need to fix.
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)