[pc86]

Airlines have dispatch folks that spend a lot of time deciding what altitude and route to file. Yes there are defaults so it's certainly possible they were busy and just filed the same but especially days apart it's likely that was the optimal altitude.

They're also looking at fuel economy so even if it takes longer if the air is thinner and they burn less fuel they may still save money. I imagine there's a certain distance where those two lines cross that is probably baked into the dispatch software for each plane model.