The overall rating is a stand-alone category. It is not distilled from any of the other categories. It is simply an 'overall' summation of the guest's 'overall' experience during their stay.
Lots of things at Airbnb do not make sense. It's a bit of a rabbit hole we hosts walk into when we enter the world of Airbnb and their ever-changing policies, procedures, and terms of service, and there are many oddities and inconsistencies which are sure to reveal themselves over time.