Why (or how) order number increases in Drupal Commerce
If you are certain there are no other users that have access to the order process, it might be caused by abandoned carts or failing payments.

Best is to have a look at example.com/admin/commerce/orders. It should give you a full list of all orders and the states they are in. Like I said if these were abandoned carts or half completed payments, orders might remain in a certain state, in which they are invisible in some other UI pages.

To answer your question. The Drupal Commerce order id is an auto increment field just as any other ID in Drupal, and it should increase with your auto increment value (mostly 1, though I sometimes have heard 5 as well).