At lower grades, and higher speeds, how fast you climb is not purely power to weight ratio; power to frontal volume still comes into play.

The steeper the hill, and the lower the speeds, the less power to surface volume matters, and it becomes almost totally power to weight ratio.

Thus if your absolute power is relatively high, but your w/kg are compartively lower, you'll be more competitive on shallower grades than you will be on really steep stuff.