@Marg11 Yes, plenty of hosts have assumed the overall is an average of the other ratings, you are definitely not alone in that.
It seems odd that a guest would give a 4* overall when they marked all 5s for the individual categories, and indeed sometimes you can't chalk it up to anything but fickle human minds.
But there are other things that aren't covered under the other categories that might cause a guest to do that. For instance, suppose they found the mattress uncomfortable, or the pillows. Maybe there were noisy neighbors, which of course a host has no control over, but might cause a guest to not rate 5*s. Maybe something about the host made them a bit uncomfortable, not because the host did anything wrong, but just has a personality that rubs the guest the wrong way.
It's really impossible to second guess, unless the guest writes something in the review that would indicate that they were bothered by something.
Don't put too much stock in ratings,- you are fully booked, I assume you enjoy hosting, and that you aren't charging too little to make it worthwhile, and that's really all that matters.