It is usually extremely hard to judge how many instances have been populated just by looking at bounding boxes. If you are looking at the actual wireframes or smooth shaded, then that is different, and would potentially show a real discrepancy.
As for why it is more sparse on top of the rocks, it could be because it is calculating the spacing based on the original undisplaced planet's coordinates, and those coordinates have been "spaced out" (space between them increased, i.e. the area has been stretched) by displacement. I would think the Compute Normal/Terrain would fix this discrepancy, but I'm not sure.
- Oshyan