[support] User ID in users table

Hi Folks In our development environment I created few users after quite long. I see the user ids are not increasing monotonically. I had around 5/6 users in development environment. Suddenly the UID allocated to new user is 26. Next user , UID allocated is 33. Can somebody tell how the UID is decided, I checked the table "users", there uid is not auto_increment one. On what basis UID is decided? Thanks kamal necs, bangalore