@Cheryl8 The overall star rating is a separate rating, not any average of the separate category ratings. This business of giving 5*s across the board and then a 4* overall happens a lot and can be for many reasons, or none that are explainable.
Possible reasons:
Guest never leaves 5*s because "nothing is ever perfect".
Airbnb tells guests that 4*s indicates "Good". So the guest thinks they are giving a good rating the host will be pleased with. And there's nothing wrong with Good in the real world, just in Airbnb land.
There was something that wasn't covered under the category ratings. Maybe the guest found the bed uncomfortable, maybe they were bothered by traffic noise, maybe they thought there weren't enough kitchen utensils, could be anything.
None of the above. Just fickle.
Don't fret over it. It isn't the end of the world. Few hosts can maintain a 5* rating, no matter how wonderful the place is, nor how attentive the host. Ratings are the way Airbnb keeps hosts stressed out and jumping to please every guest, no matter how entitled or demanding they are. Don't get sucked into it.