Let me echo what Terry and Jeff have said. If you rifle does not fit you well you will not do well with it. Try shooting a rifle with a length of pull that is too long for you, for example.
Another great example is indeed the grip on an AR-15. The standard grip is way to small for me but does not really deter me from shooting it fine, but the grip shown above actualy hurt the side of my hand as it placed it in an unnatural position when prone. Also, I don't like finger groves, because they are never right for my hand and the grip itself is skinny. That was not conducive to good accuracy.
The grip that I have been using for the last several years is very nice as it fits me like a driving glove. It just adds to the general fit of the rifle where I simply don't think about it because it does not fight me.
I think that helps my scores.
