Why (or how) order number increases in Drupal Commerce Does anyone know how the Order number increases in Drupal Commerce?

Obviously when someone puts in a new order the order number will be AT LEAST one larger than the previous order number, but other than that why would the order number increase?

For example while testing the site my first order # is 1 (duh), but then 19 days later when I tested it the order # was 19, without any other tests occurring in the interim. The following day I did a test transaction and the order # became 31. 

It keeps going up every time I test it but it doesn't just increment by one every time. So I'm just curious why it might be behaving this way. Anyone know?
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).