@Emma501 The individual star ratings for cleanliness, etc, are not averaged to get the Overall. The overall is separate- and they have recently changed the guest review form, so that guests don't even see a star ratings for Overall- guests have boxes to check- what you got, a 4*, means the guest checked the box "A bit better than I expected". Should be good, right? Nope, not as far as Airbnb is concerned when it comes to hosts.
Hosts have been up in arms about this sneaky, non-transparent review change- search other lengthy threads on the hosting forum for more info and host responses. "New Review Form".