I don't think it's ever been officially explained, but I've always assumed it's worked like this:

The red dots are (hypothetical) clay nodes, which are randomly placed all over the map (regardless of terrain). A clay node might be centralized in the middle of a lake, and you'll only be able to notice it by digging the outskirts of it on the shallows of the lake. As the distance from the center of the node increases, the quality of the resource found decreases, although not necessarily at the same rate for each node. The center of each node yields the highest possible quality.
This is all purely conjecture based on how I've experienced the resource nodes in the game, but it seems reasonable enough.