It's definitely weird. There was a job I knew about that hadn't been made available to me. (A variation of one of the shops I frequently WAS offered.) Then, voila! Came to me in one of those desperate "we need a last minute shop" emails. Unfortunately, it wasn't in an area close to me, but I seized the opportunity to tell the scheduler that I want the shop when it IS closer! To make a longer story shorter, the scheduler wound up calling me and we sort of negotiated -- I agreed to help her out with one of the desperation shops and now I am regularly offered the shop I wanted. They don't give out grades, but the scheduler told me I was a "good shopper" so they definitely do something internally.
There is a method to their madness, but I really don't know what it is! I know there are other shops lurking about, which still appear to me infrequently...kind of makes me wonder if certification does make a difference with this company. No clue.