You forgot to tell him why this is so Wouter
I am a great believer in knowing why some settings are fast, instead of just know that it works.

A flat airfoil has less drag than one with more draft. In light winds the lift to drag ratio for a flat airfoil results in a larger vector forwards than a deep one. When the wind picks up, the ratio between lift to drag from the deeper airfoil makes a better resultant vector forwards. At least, this is my understanding of it.