It would help to know the route and the operating carriers for the OP's award tickets. Otherwise we can only guess whether the amount charged could be taxes and fees.

OTOH, if OP has the emailed ticket receipt(s), looking at that carefully might solve the problem. Also, if DL charged in both money and miles, there might well be duplicate reservations and more importantly duplicate tickets with duplicate receipts. Almost all award tickets carry at least a small charge in money, so the OP should check whether DL has charged a small amount and a large amount to the credit card, as this would also indicate that the flights were ticketed both for being paid in miles (award tickets) and paid in money. I would assume that it would be difficult for an error to occur that would mean that the OP has paid for the same tickets in both money and miles, although of course some "pay with miles" tickets could look this way if one weren't paying attention when selecting how many miles to apply toward the fare.