This actually makes a great deal of sense. People are stupid, generally speaking.
AMD gave tons of Ashes copies away with their hardware, and many of those owners got on steam and bitched the game out, primarily because it wasn't their cup of tea. These probable non-customers, who aren't likely to enjoy a particular game in the first place due to the simple statistical realities of personal preference in regards to entertainment, skew the practical reality of a review average. It knocked around 10 percent off the score if I remember right. The only decent game I've ever gotten with hardware was Shogun 2, everything else was something I had little to no interest in, and a few of them were extremely popular games.
When you're going to buy a game, you don't care what everyone's opinion is on the game, it's meaningless. I'd say the best visual novel ever made is retarded shit and people should go read a real book instead(or watch porn, depending on the content), but I'd never actually buy one because I don't like them. Someone that does like them, because they have severe brain damage and enjoy such silly things, wants to hear from other people that like them, not me. I'm just the asshole insulting them, not someone giving a valid opinion on whether the game is a good representation of it's concept.
People are also being "paid" for reviews, by being gifted a steam key from the producers, which may or may not result in accurate reviews, and might just be score padding at release. Unlike an actual reviewer, where the content of their review results in their reputation taking a hit if they're a shill for the company, Steam's review system is almost entirely a function of the aggregate and such deceptions remain largely unnoticed and impractical to account for.