He's saying the pressure on the barrel from the synthetic stock is pretty constant shooting from a fixed rest. That pressure will vary when you're gripping the forearm, and so will your groups.
My Rem 700 SPS shot consistent groups with the factory stock... consistently shotty that is. I put a laminate stock on it, hopefully that will fix the problem. Haven't shot it yet... its cold out.
The good news here is that I can sell my crappy stock to someone else and for $50! Woohoo!