Device ACCOUNT number

If you go to the Wallet app, not trying to open Apple Pay, bring the card up, and then tap on the ℹ at the bottom right area of the page, it brings up the card information. Under the area Device Account Number, you will see an explanation of this being a generated number that Apple uses for transactions rather than your credit card number.

Under what circumstances did you see a different number? If you turn off your passcode or other security, it will have to generate a new number. If you get a new phone and add the card, it will generate a new number, and those are just what I am aware of. Not sure if there are others.