I have 110 in adventuring, I just purchased the generic large and medium backpacks. Just for fun I tried to use the large which requires 180. It opened and could be used without problem.
I tried to put it on my back, but could not.
What am I missing, what does it matter if I cannot put it on my back? It grants nothing. Maybe it needs to be on your back to expand the amount of weight you can carry but I have a level 88 who has never worn a backpack, just kept them in the inventory. Never had any problem with too much weight.
Bug? Not going to waste 70 in adventuring for 1 inventory slot