I've never really noticed it before, but as I walked my last Pyrenees walk I noticed that there were quite a few people hiking along with the same rucksack as me, an Osprey Exos 48L. As I looked as they walked ahead, or passed by me, three positions seemed to be the norm. Like me, some tightened their belt just below their hipbones, some tightented them half above and half below their hip bones and I was surprised to see that some people even tightened them above their hip bones, just relying on their squidgy fleshy waists for 'grip'. Some looked most uncomfortable.
As regards the shoulders straps, I wear mine tight and I make the rucksack sculptured to my body shape (I don't want any movement, the rucksack has to be part of me) but some people I observed had yawning gaps between their shoulders and the top straps of the rucksack and their bags wobbled as they walked. Again, to me this looked most odd, uncomfortable and if clambering up or down a steep section they I believe could easily become unbalanced.
Maybe I'm the one doing it wrong?