The only non-stop DL service ORD-AMS is the KL flight code-shared with DL. The other DL services you see are the usual robbery practiced by US airlines selling so-called direct flights (as opposed to non-stop flights) which in fact involve a connection in one of the hub (ATL, DTW, MSP, etc); with the 2 flights bearing the same flight number. But it is a real connection with a change of plane and crew.