Using grounding clips

The Unirac Code Compliant Installation Manual, on Page 25, shows that you should use two grounding clips between modules, at every other space between modules.  Ironridge recommends that you specify one grounding clip for every two mid-clamps.

I'm curious why we can't use only one grounding clip for every four mid-clamps.  This would allow us to ground two modules to one rail, then use grounding lugs to take the ground from rail to rail. 

Do these manufacturers suggest you use an extra grounding clip for redundancy, or is there an actual electrical need to double up the clips?

Thanks for all your help!