Profit. Plain and simple.
Service and Parts are the only "real" profit generators at dealerships. Some stores will be worse and some better than others. We are usually pretty spot on with whatever Subaru has for pricing.
There have been times where I would go to Parts with a customers request for something that I had quoted per Subaru and my stores "price" was higher. I would then make them honor the price I had already given that came from Subaru directly.