I use a number of different processors. One thing I do notice is there is a difference in the way they process an order.  I always specify maximum trim and minimum bone.

There is a difference in how much time and effort they put into the trimming operation. Lockers get paid on the hanging weight not the final in the box weight, so there is not much incentive for them to get all of the trim.

Not to say there are not some shenanigans going on, but I think much of the difference between processors is just the time and care they put into the job.  Some have more scrap than others.

If they are busy they just don't spend the time to get it all, especially since they are paid the same.

jmho and experience.

Jim