From GameWatcher: "Valve have changed the user review system on Steam to show whether a game’s experience has changed months after being released. The system now uses the newest reviews to calculate a summary. The old reviews are still there, only now they are shown in the overall summary.
This new system identifies two problems that used to occur and deals with them by: displaying recent reviews more prominently and by including a ‘Recent review score’ summary. This allows Steam to display an accurate representation of the game in its current state."