Having spent much time watching a very well respected brand grade it went something along the lines of provisionally marking out grades based of looks, weight would then lead to bats being swapped a little, heavier going down and lighter coming up but only ever one grade. Finally every bat was hit with a mallet and nothing was ever upgraded but some is the highest bracket could drop to the bottom one. Why? well because to be a top model it needed to look and play the part so everyone spending a lot of money was happy, but in the middle you could get something that didn't quite look the part but played as well as the top one. In the bottom grade it might look good or bad but did not quite ever play as well as the other 2 grades. To me is was and is a very fair method of grading.
However you then get the buyer who see's 14 straight clean grains in the bottom grade and claims to have found one that slipped through the net and promptly removes the grading sticker and pronounce s a bargain to be had when they sell it on. Who benefits from this? purely the seller.