Why my order IDs look like #1000X after migration to Magento?

It is due to the Magento platform peculiarities that order number is built in a specific way. The first number is the ID of your store, the following is the order ID. All new orders in Magento platform will look like #1000X. For instance, Magento order #100035 where 1 is the Store ID and 35 - the incremented order ID. That’s why when choosing the option Preserve Order IDs all the previous orders will be migrated after Magento pattern, i.e. #X - > #1000X for the proper system functioning.