I have noticed that and how the ideal position of the paddler on the seat appears to effect what the rocker is there for. I found this interesting concept on the Outrigger Canoe Design site:
The main problem/reason is, that most boats are hand designed. Therefor it is almost impossible to scale to certain weights.
A less weight sensitive boat is designed flatter so that the bow and stern are more in the water. That way a lighter paddler paddle won't lose the length.
For heavier paddlers rockered boats are better due to the rapid increase of volume with added weight, less wetted surface is induced that way.
Now it doesn't say for what conditions the rocker is better but that it is better. I have found moving my weight forward in the flats on my Zephyr helps get it running smoother and what "feels" like better glide.