Please explain me about how the average score at reviews is calculated.For example if all scores are about 10.0, how can the average be 9.0 OR if specific scores are between 5.0 to 7.5, how the average score be 1.0 ?

There are two separate scores that guests are invited to give. Firstly the group of scores for location. cleanliness, etc.

Then there is an overall score they can give that is independent to the other group of scores.

This means a guest can, for example, give you straight 10s for every category than give you a 1, if they so wish, for your overall score.

It's a pretty naff system but BDC have refused to change it.