Pitt get's calls in their favor all the time.
I think they have a large fanbase and included in that fanbase is referee's. Probably ref's who grew up in the 1970's when the Steelers were winning championship's with their heavily juiced up players. Now, still fans, they stick a call or two in their favor when they get the chance, maybe even subconsciously. I know if I were a ref, I'd probably slight calls towards the Ravens.