Particles are not in contact with object I've been watching Andrew Prices tutorial on "how to make a cave in blender". So now I'm trying to put mushrooms in the cave as a particle system to spread them along the floor of the cave.



Unfortunately, as shown by the red arrows, some of the mushrooms are not in contact with the cave at all, and are hovering either above or below the cave floor. 

Is this some kind of setting that makes particles placement random? Anyway, here's the .blend file if you wanna take a look.

Also, I wouldn't mind if I got some overall feedback too!
It seems it is the child particles which are being placed above or below the surface. I found that changing the 'Children' type on the particle system from 'Simple' to 'Interpolated' created a more accurate result, though I'm not quite sure why 'Simple' children wouldn't respect the location as much: