@Gail84 The Overall rating has never been an average of the individual scores. Guests are prompted to leave feedback when choosing a score other than 5 stars, but they don't always choose to do so.
The likeliest reason for choosing the seemingly mismatched Overall score is that the guest had an issue that doesn't relate to the individual categories. It could have been something about the beds, the temperature, the home decor, an odor, insects or spiders, or some other detail that didn't meet their expectations. But although Airbnb pushes 5 stars as the only acceptable rating, most people genuinely believe that 4 out of 5 is still a positive score. It'll drive you crazy if you let it, but hosts who fully ignore the stars and badges and all the silly Silicon Valley stuff can get every bit as much out of their work as hosts who let themselves get obsessed about it.