Originally Posted by merlinextraligh

In hot weather, I think you have a point.

If its not hot it should essentially be equal. Watts are watts. From a mental standpoint, its easier for the vast majority of people to hold a high wattage level for a sustained period on hills than flats.

Go up a 10% grade, it may take 300 watts just to maintain a decent pace. It feels natural to be working that hard.
The hill pushes you.

Now try to maintain 300 watts on flat ground for a sustained period, and you have to push yourself to keep working that hard and going that fast.

This is why the wattage targets for climbing intervals are typically set a little bit higher than the wattage targets for steady states of the same duration on the flats.