@Natalie1176 Since whatever you say now is really pertinent to only future guests, definitely not the time correcting past guests or their thinking publicly, the best course of action is to act like you just got a good review, emphasizing whatever positive they happened to mentioned.
As for why that guest left that who knows, and happens often. Perhaps is in the methodology of rating, misunderstanding by the guest or even outright mistake. If you suspect is the latter, ask the guest to change it, otherwise is not worth worrying too much about it. What guest say is a lot important then one's star rating, which could goofy at times.