I know I’m late to the party but I just found out and didn’t find a post regarding this
TL;DR: Steam now calculates review scores based on reviews made on your languages only (if enough). This is on by default, but can be reverted to go back to review scores based on all reviews (from people that purchased the game directly on Steam only)
I’ve done that and it’s a result of not having more options than good/bad. Always the same cause: I really wanted to write a 3* review for a game that has a lot to praise but its core is fundamentally flawed, but Steam doesn’t let me give a 3*, so I try to correct for the review score bracket I think the game should be in.