So it seems you are doing a roundtrip search. Is this with ATL origin or AMS? It's hard to know what is being quoted here without seeing flights in both directions of travel on which the total price is based and fare classes involved. The pricing differences here don't seem all that extreme and it looks likely that VS has inventory in one lower fare class on a flight (such as Z or I) where DL only has inventory in a class higher (like I or D). Do the Amex non-discounted prices show the same on Google Flights?