Oddly, they aren't so much sagging as there are high spots in places. Is is a long seam between sheets of plywood and they may well have been placed too tight together and they grew a touch and humped up. Gotta have room for wood to move!
When we removed carpet in the first bathroom I did we found a few chunks of cedar shake that were used to level the floor in a doorway.
We have high spots throughout this level. We can feel them under the carpet. It really is the bad.
Joists are 2x10 on 16 in centers. The low spot in one doorway is less than 3 feet from a load bearing wall, concrete block no less. I feel less that we have structural issues than we have a house framed by morons. Bet a few joists are crown up, some are crown down.