The short version is that the Trustscore is an average that takes representativity and the reviews' "age" into account.
The TrustScore is not simply an average of individual user ratings.
It attempts at giving a mathematically sound approximation of the probability of having a satisfying experience when shopping with a given company. One thing it does to achieve this, is to never allow a rating to deviate too far from the average, unless we have enough data to soundly make bold claims about the customer satisfaction.
If there are only few reviews, the score will not deviate a lot from the median, 7.
As more reviews are written, the formula functions more and more like a weighted average, where new reviews count more than old reviews.