You are certainly the pro and I am less than the average joe but I thought I read that on the tag or whatever that came with my stihl chaps? When I got them they were pretty flat but then after I washed them they kind if puffed up a bit. I think when the fibers are washed they become finer or more frayed and puff up like after you wash a pair of Jeans after you cut the legs off making them more effective if hit with the spinning chain?
Again a theory. I am 99.9% sure my stihl chaps said to wash them prior to use. I just observed them before and after I washed them and came to that hypothesis.