I am wondering , since it is the same game why user score is so different .
Why do you guys think ?
Cheers,

Audience.

Mainly those review scores differ due to the audience. Steam has a bigger platform with most likely more players of the game. While GOG is known for a more sophisticated audience who are veterans in this kind of games and not so easily to impress considering the good old games of the same genre.

It matters also what kind of audience is taking more time to review something and how they do it. A five star rating is absolutely insufficient to say that 4 stars rating is better than a 3.5 stars rating. It might look different if you have to give it points out of hundred.

But then again you can check the reviews and see what kind of audience is writing actually what. If they wrote anything at all. Even the press reviews which are reflected on GOG is changing on a semi-daily basis. At launch it was 90/100. Then on some point 80/100. Now it is 83/100.

The answer to your question is basically the same why different games magazines give different ratings.

When you look at GOG and Steam reviews, examine them individually. Some love the game, some hate it, some are lukewarm, etc. I don't know how Steam and GOG compile their aggregate averages, but that is done differently as well between the sites. It would be time to wonder if the averages were the same, instead of being different, imo...;) Different strokes for different folks--it's as simple as that.