I don't pay attention to weight range either. I have a skinny dd and whenever I size up on anything (because 'in theory ' she should be coming up to that size) I end up with a nappy which is way too big.
I bought a couple of large WNNN recently as I thought they would last my 12 month old longer. The rise comes to above her belly button and she can wee out of the leg because they gape so much! Hmm...they have had to go to the back of the cupboard.
It sounds like there are plenty of babies that don't get to the large sizes before PT. Just go with what feels right for your lo.
BTW I have a couple of large Sandy's. The legs are too loose but under a good wrap I have not had any leaks x