On Steam and elsewhere, a game's release is not necessarily the end of its development. With patches, DLC and other updates, the quality of a game changes over time, especially when a game is in Early Access.
To reflect that in the user review scores, Valve has announced that it's making an important change to its user reviews system on Steam.
"One common theme we've been seeing in customer feedback about the Steam review system is that it isn't always easy to tell what the current experience is like in a game months after release. This new set of changes released today is designed to better describe the current customer experience in those games. We do this by better exposing the newly posted reviews and by calculating a summary of those recent reviews."
The most recent reviews will be calculated over the past 30 days, "as long as there are enough reviews posted within those 30 days and as long as the game has been available on Steam for at least 45 days." The recent review score does not replace the overall score, which is also shown alongside with it, so users can still see how a title was first rated.
Another enhancement to the user reviews system is the addition of a checkbox for people writing a review to easily disclose if they received the game for free. You can now also view all reviews regardless of language by selecting "All Languages".
If you have questions, feedback, or find bugs in the review system, you can make yourself heard in the Steam Forum at http://steamcommunity.com/discussions/forum/0/