The guest gave me a review of 10 points, but the average calculated by Booking is 9. Why?

I received a review from a customer who selected a grade of 10 for all points in the questionnaire (Staff, Cleanliness, Location, Facilities, Comfort, Value for money), but the grade calculated on the site is 9.00. Why? 

Ismini Karali 2 years ago

The system has changed for more than 2 years now.

The guest can rate all 6 categories, and then put an overall score from 1-10. (completely unfair, but still have to live with it..)