I was looking for an explanation based on Physics .
Thanks for the replies.
In terms of studying for Air law, I think the airquiz site is excellent for learning the subject material. It identifies what you don't know; you can then simply learn the answer.
The argument of not understanding the subject material does not hold. If you identify that you don't know what 2 red spheres hanging from a mask signifies, then identifying that it means gliding is in progress is all that is necessary, there are no underpinning hanging ball theories that must be mastered to apply the knowledge to a practical situation. I agree for other subjects that airquiz might not be the way.
I was looking for an answer based on physics since at first sight I thought the altimeter should under read since cold air is more dense. But I think I have now managed to apply the physics to the problem which seems to fit with the correct answer.
Anyway I think the best that came out of this was the post by whirlygig, that will make be remember the important fact.