Well, believe it. Because that IS what is happening. All you have to do to be able to believe it is to consider how many coins there are sent by dealers and collectors alike that come back with a lower grade than what they expected to get. Almost every single one of those coins is a loss for the submitter.
It all boils down to a numbers game. You send in a batch of at least 100 coins in no more than 4 different dates/mint marks and hope you score enough higher graded coins to cover those which grade lower. Generally it works out in your favor provided you cherrypicked the submission. You blow out the lower graded coins at or just below cost and let the higher graded coins do all the work. It's a big money maker for the larger coin houses.