Each Shopify order has a unique Order identifier (ID and Name) and a Checkout identifier.
Order ID and Name are usually the preferred Transaction ID values, making it easy to find and compare related transactions.
However, Shopify Order identifiers cannot be reachable during the express checkout process and cannot be used as Transaction ID values.
Since the Transaction ID field cannot be empty/null, the Checkout ID value is transmitted as the Transaction ID value instead of the Order identifier in such processes.