User uid incrementing in non sequential manner
If it is exactly 10 then it might have to do with MySQL's Auto Increment value
https://stackoverflow.com/questions/1686327/change-the-step-auto-increment-fields-increment-by

drupal.org currently uses this feature for master slave replication. Right now all users and nodes being created have a odd value. If you look further back this value was EVEN at one point.