Skipped order numbers in the web store order queue should generally be no cause for concern. The next order number is a dedicated value. So, if a user has created an order and it has not been processed, then that order number is locked and skipped for subsequent orders.
For example, if a user is checking out with an order and finds that their credit card has insufficient funds, then the order number has been used because it was sent to the credit card gateway. As such, it cannot be used again.