This question is likely better asked at http://gaming.stackexchange.com/
However, as 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.