I wonder if hip angle influences this feeling at all. Going downhill, one usually sits more upright, which is a more open hip angle.
Going uphill, there’s usually more bodyweight over the front of the bars for climbing and other reasons, which would make for a smaller hip angle.
But that seems like it would run counter to @Jonathan ’s point that power is easier to generate uphill than down…
I don’t know, but I’m interested to find out!