I think the system wants the traveler to verify a passport at the airport, regardless of whether the info is in your Delta account. There are multiple checks to ensure that someone doesn't board an international flight without the required documents. In a similar redundancy, you have to show your passport as you board an international flight even though you have already verified it at a kiosk during check-in.
Some years ago I was on a DTW to Europe flight where during the boarding process a passenger realized she had lost her passport somewhere in the airport. She had used it to check in and then lost it. Her problem was discovered at the beginning of boarding because she and her husband were in J, so everyone saw her frantically searching around the boarding area. She hadn't found it by the end of boarding, so she and her husband remained at DTW as the flight departed. That episode convinced me of the necessity of the multiple passport checks.