The higher the leg goes up, (providing flexibility is not a restricting factor), the easier the leg extension becomes.
Confused? Pick up a heavy object, and keep your elbow straight. Extend the straight arm parallel to the floor. (Hand on the same level as the shoulder) Difficult to hold, right?
Ok, now bring the arm straight up. (Hand holding the object directly over your shoulder) Much easier, right?
This is because the first position, is mechanically more disadvantageous. Second one is more advantageous.