Could be just a QC issue. That's why they have Lot and Reference numbers. Cone mentioned in one of his blogs about their Photo Rag being different with bleeding and maybe QC issues from the factory. Old paper may get more stiff and brittle too.
I have some that has a small 6mm border near the entire edge where it is finished differently than the middle of the roll, like it was the cut end of a loaf of bread or their texturing rollers didn't go off the edge. Not a big deal as I don't print borderless, but still annoying to get that side roll piece.
SG