http://store.steampowered.com/news/24155/ states:

> As of today, the recent and overall review scores we show at the top
> of a product page will no longer include reviews written by customers
> that activated the game through a Steam product key.
> 
> Customers that received the game from a source outside of Steam (e.g.
> via a giveaway site, purchased from another digital or retail store,
> or received for testing purposes from the developer) will still be
> able to write a review of the game on Steam to share their experience.
> These reviews will still be visible on the store page, but they will
> no longer contribute to the score.