I think there was a discussion about this in the IAEI magazine a while ago. It has to do with the thermal conductivity of the wire as I recall. If I have an area where the ambient requires derating I can cheat for the first 10' but if that gets to the termination, I have to respect the termination temperature.
... or something like that.
It never really made much sense to me, particularly when they got so excited about an inch and a half of Romex in spray foam. 334.80