Just like flying over water, all grass and nothing to break up the patter or lack of, can cause the depth perseption to be questionable. I have caught myself staring at the ground before flare and wondering where the ground was. look up and down your landing path. building and shrubbs both near and far can give you better perception then just grass or dirt to look at.
In planes you look down the runway not at it so why would canopy landing be any different(curious)
If I am wrong feel free to tell me.