Particles not stciking to vertex group


The brown areas are set as a vertex group. The little rocks are set to stick to the subdivided faces of the vertex group. But as you can see, some the little rocks are going outside the vertex group. Any idea why this is happening.

win 7 blender 2.77a.



Particles will spill over to adjacent faces. Easiest way is to remove the perimeter verts from the vertex group.