Possibly because it's something they can't really put into words. Some bodyshop guys just have their own special ways of prepping.
I prepped an old mini years ago. Bodyshop guy told me to keep going at it for almost a year before he would accept it fit enough to paint.
What one person sees as acceptable, another wouldn't. Totally unacceptable is very obvious thankfully.
Another tip: If he carries a white stick, you've got the wrong bodyshop.
