For me it's basically what has been stated before. When a big name game comes out it seems like they try too hard to find points against it so they can seem unbiased. If it's a new game or one that sort of comes out of nowhere, they feel more free to give it the score they really want because no one would accuse them of being biased towards such a small publisher, value priced games, etc. It's just safer for them.
It's safer to not rave over the 'big event' games and champion the more unheard of titles?