When the transactions 'arrive' in the bank's system should be immaterial, as they should be timestamped at the moment the card is swiped, matching the date and time the printed receipt I am handed has.

Regardless, the bank's mission is to make money, and they can make a lot more of it by adversely reordering transactions in their favor, it is not at all a reach to assume that they do so, and in fact have done so and may well continue to.