Valve has taken measures to update Steam’s reviews platform so that user reviews will now “better describe the current customer experience in games.”
The game must also have been on Steam for at least 45 days.
You can see a full log of changes as well as Valve’s explanations here, but this should just make things easier to wade through if you take Steam reviews or any reviews for that matter seriously.
There’s also a new “Recent review score” that will appear on a game’s page that offers a calculation of review scores within the past 30 days, if there are enough reviews to provide it.