You may sometimes notice out of sequence Magento order numbers and wonder if perhaps there are problems with the website. Out of sequence order numbers in Magento are to be expected (although if you do see a sharp increase in the amount of orders being jumped it may be worth looking into further), but the below are the two most common reasons why you might see out of sequence order numbers in Magento.
1) A logged in / registered customer didn't complete an order. After a critical stage - typically at the payment method step, an order number is then allocated to the quote (which can never be used again). If the customer then comes back at a later stage to complete the order, the allocated order number will appear out of sequence (e.g. 100000004, 100000002, 100000005).
2) A guest customer adds items to their cart and passes a critical stage of the checkout. At this stage, the order number is allocated to the quote and if the guest customer does not check out before their session expires then their order number will be lost and not used again).