Originally Posted by merlinextraligh
I think, its mostly mental. Simply maintaing a decent pace on a steep climb mandates putting out high wattage.
It takes a significant mental effort to crank out high watts on the flats.
+1
In addition, on a hill you can't coast like you can on the flat...well I suppose you could but then you might fall over at some point.
I notice the same thing myself. If I do 20' field tests, they always come out better on a hill compared with the flat.