Quote from @Toni Escuder:
I actually read somewhere recently that hosts should aim for a review average between of 4.6 and 4.8 ( or something like that) because only 5 star reviews means you are leaving money on the table and giving in to guests demands too often.
Most places I see in this range don't look so great and a quick scan through the reviews makes it easy to find what they are doing wrong. I would also disagree with the "demands", I can't think of any situation where I have given into a demand or unreasonable request and my rating is 4.99 with 70+ reviews.
That said I do agree that perfect ratings set off alarms for most, there have been studies with Amazon reviews about this. I would also agree that you probably don't want a 5.0 in the "Value" section on Airbnb, because if the guests think they are getting a deal then you are probably leaving money on the table.
As for the OP, apologize if it is a legitimate grievance and you are at fault, then explain how you will remedy the issue for future guests. If it's absurd, point out what was good about the place like the Pool Maintenance comment above. Keep it short and sweet. As time goes on, the bad reviews will sink further and further back and your service will improve.
Also- helps to check-in/follow up and humanize yourself with the guest throughout the process from post-booking through their stay. Especially important to send a message shortly after they check in to ask if there's anything I can do to make their stay better.