Looking at http://www.gog.com/gamecard/raptor_call_of_the_shadows_2010_edition I realized that you either weight the different opinions according to a formula or just round the average up. The arithmetic mean would be 4.13. Shown is 4.5 stars. That explains a small part of the inflationary ratings on GOG.com.

Also the amount of up-voters per review is preferred to the ratio of helpful to not helpful which can lead to mass preferred to quality.

How do you know how people individually rated this game?

You don't need to post a review to rate a game. There are 1523 ratings there and only 68 reviews.

The average rating is based on ratings, not reviews.