With our new move to a replicated MySQL server environment, we had to change the
increment value to 20. This allows future growth when dealing with master to
master server replication. So this would explain what you are seeing.

This link explains some of the reason it must be configured this way.

http://www.onlamp.com/pub/a/onlamp/2006/04/20/advanced-mysql-replication.html

It is also well documented in the MySQL documentation but I can't find the link
at the moment. We plan to have multiple masters spread around geographically in
the future. The gaps you have in your primary keys determine how many masters
you can have so currently this would limit us to 10. We used to have this set to
20 and 
since lowered it to 10.