@Anne420 The overall rating is totally separate from the category ratings. When a guest chooses not to assign a category rating, it defaults to 5 stars regardless of the Overall, so that's one thing that might have happened. Another possibility is that whatever issue they felt was not-quite-perfect was outside of the categories Airbnb gave them to rate. Like maybe they were satisfied with the location, cleanliness, etc but didn't like the home's temperature, or found the bed uncomfortable, or the chickens too noisy. I've had a few like this where the only thing they had to say was that our building was "old" - as if a good host would tear their house down and build a new one every year to make sure nobody was troubled by the presence of history.
You can always follow up with the guests to ask for feedback about what you could do differently to deliver a 5 star stay the next time, but considering that you already have hundreds of excellent reviews, that might be overkill.