I too am curious. Usually they go from China directly to Alaska with no stop in Korea.
Unfortunately, I don’t recall the routes taken by the Apple products I’ve ordered in the past, however, I find it strange that after arriving in Korea on the 19th, my iPad Air remained there for almost three days before failing to clear customs.
According to the Wikipedia page on the Chengdu airport, UPS does not have any flights to Anchorage. However, Fedex does. So perhaps this explains why UPS flew our iPads Air to Incheon (Seoul), from where UPS does fly to Anchorage.