I was talking to my LHBS Manager and he was saying that depending on how full or empty a grain sack is when they are parcelling it out in bins for the shelves will effect efficiency.
He was saying that if it is the last 10-15% of a sack, there is a good chance that the ratio of hulls to grain shifts unfavorable towards hulls since the grain gets bounced around and the hulls will settle to the bottom. He tries to fight this by mixing sacks up once they are 50% gone but says it still is an issue that arises.
Maybe some instances of bad efficiency could be related to this. But in terms of crush... I always send it through the LHBS mill 2 times to make sure I am getting the best crush I can get out of there.