because the review system on gog sucks... even gog support cannot change the star that you give to a game (you can only change the review).
Example. The Witcher 1 mac version some times ago was an old wineskin port, the performance were really bad... I had to port the game myself with a new port with playonmac using the windows version. I gave 3 stars: good game but terrible performance on mac.
After that they updated it with an eON (almost native) port, with great performance. I had to contact gog just to change the text of my review but there is no way to give 5 stars.
Doesn't Steam allow you to edit reviews.
yes, like any other digital store.
since they can update we can change our mind about a game, it is normal.
Still on steam you need to play at least 15 minutes to review a game, here you can even review a game that you don't own (I'm not against this, both make sense... but I would prefer something like amazon where everyone can review but if you bought something your review say that).