I imagine it has something to do with sparse tracking. The final ascent / decent is I think the better part of an entire day. You can't have full high accuracy

I imagine it has something to do with sparse tracking. The final ascent / decent is I think the better part of an entire day. You can't have full high accuracy GPS on for that long and have the battery last. So you may put the device away for a while, and get it back out when you finally summit to track that you were there. And lower on the mountain you might have extra ridges in the way, like in a city with lots of tall buildings, where you just can't get the best signal. And on Everest there is no WiFi or cell signal to cheat with. I would also think that personal device GPS may get a bit confused at that altitude. That is not really the expected working conditions.