Grobb,
Ok, I got your issue now.
Agreed that the true altitude is decreasing and you are going lower down in the atmosphere. BUT, that low pressure up there that your heading too has been created by a shed load of cold air around. Dick is correct in saying that whilst you may be descending and the air is warming, the overiding factor is the colder mass air your about to fly into.
That is what the JAA is testing, and it's our experoience as instructors that enable us to know what concept they are trying to test, sometimes the JAA do it very badly and even we don't know what they are getting at but most of the time the questions in MET are well written.
Pugs