Because as the air gets thinner the thrust on the remaining engine reduces. This makes the asymmetric thrust easier to control.
As to your observation, I use this in class to explain my statement above although, as you can see, there's much more to it.
The determination of Vmca also allows more freedom of manoeuvre. Vmca is determined so that the heading change during the recovery from an engine failure at that speed is not more than 20º whereas Vmcg has a more restrictive limit of no more than 30ft deviation from the centreline.
Last edited by Alex Whittingham; 14th November 2002 at 17:34.