why are some order numbers missing from the online store?

Hello Karla,

There are a number of possible states for orders including those that were never completed by the customer. There are also things like cancellations and re-orders which can affect the appearance of the system.

Suggest that you have a look at the Ecwid KB Orders and see if this sheds possible light on the issue?

I daresay your concern is with possible over-looked order? and this is something that you need to monitor for sure but the easiest way is to do as you are doing by checking the order history and copies of emails sent to you from the payment gateway and just make sure it does balance.