I know it must be making you feel absolutely livid and we all feel for you - many of us have been there! It seems so illogical, but from a guest point of view I can see why it might happen. You know, you stay somewhere and you can’t fault the cleanliness, location, communication etc, they’re all fine, but there’s just something … you can’t put your finger on it … but it just doesn’t spell 5-star to you. This is often no reflection on the host, it may be some idiosyncracy on the part of the guest which wouldn’t be relevant for anyone else - say your place was quirky mid-century modern and they preferred beige minimalism, or, like a very picky friend of mine who won’t give anywhere 5 stars that doesn’t have 800-thread count sheets!
Some posters here have suggested educating guests that in Airbnbspeak 5 stars means absolutely fine rather than total perfection, by including something to that effect in your House Manual. You might want to try that, but I bet when you have a few more 5 star reviews under your belt you’ll just shrug and say, as we used to in the North of England “There’s nowt so daft as folks …”