You are mistaken. DL 129 involves a change of aircraft from 757-200 on ATL-SEA to a 763ER on SEA-PEK. A direct flight involving a transfer from domestic to international will almost always involve a change in aircraft.
Hmmm. I didn't know that. I had a business guest come over from ATL recently, and he really wanted the SEA flight because he was under the impression that he wouldn't need to get off the plane. He ended up doing the NRT thing instead because there was only one biz class seat open on the SEA flight, and he didn't want to abandon his wife (or, more likely, himself) in coach.