How are CiviCase Case ID numbers generated?
I am not 100% sure, but there are some peculiarities within the case process. One of them is that the complete set of data is saved at the end of the overall case transaction, and for CiviCRM purposes it is a rather long transaction. And if there is either an error or a cancel somewhere during the whole sequence it might be that a record was created in civicrm_case (allocating an ID) which is then removed later on, but the ID is gone. 

If you look at the table it does mention that ID is auto increment, meaning it is sequencing. As a test you might want to try creating a case but then not completing the open case activity and see if it allocated a case ID (if you catch my drift...) We certainly encountered some un-characteristic stuff when creating custom extensions for CiviCase :-)