PolitiFact is biased because Republicans lie more.
From the ironically named Conservative Fact Check:
The results are, sadly, not surprising. But the situation is grave. PolitiFact (and other supposedly unbiased fact-checking sites) paint Mitt Romney as a serial liar. They also unfairly tarnish Michele Bachmann as a liar, when anybody who follows her already understands that many of her statements aren’t meant to be truthful in the first place — she simply says what she feels.