Never heard of it???
Maybe its since you are flying above the earths surface (ground) you are flying a bigger curve than if you were on the ground for the same angular displacement from the earth's centre, so for the same track miles on the ground you fly a longer distance at altitude...
Therefore your speed to cover that distance in the same time must be greater than if you had to fly that angle at the earth's surface.
Managed to confuse myself too...