Your suspicion is right Stu. Regulations specified that the HJ/DJ wore the Überfallhose as part of the normal winter uniform and for skiing. There were plans to introduce special skiing trousers (Keilhose) but the regulations state that this was indefinitely on hold due to the war.
The label in some of the Überfallhosen is marked as "HJ-DJ Überfallhose Skitrikot" or "Überfallhose - Skitrikot". This may have lead to the confusion but actually it doesn't mean that they are a different item to the standard trousers.
'Skitrikot' is the name of the cloth and its use was prescribed by regulation after a certain point (not sure when exactly but it looks like either 1938 or 1941). After that certain point all HJ/DJ winter trousers were made of the same cloth which is why we see some trousers marked with 'Skitrikot' and others (the older trousers) with the standard label. It made sense I suppose to make all of the Überfallhosen out of a material more suitable for skiing in case the wearer needed to use them for that purpose.