"There is no explanation given in the feedback or review as to what they were unhappy with and they have marked each category with five stars, so what on earth was the problem?"
@Huma0 It is completely frustrating , but 4 stars does not mean the guests are unhappy, it means Airbnb is unhappy and that unhappiness is passed onto the host. The problem is that Airbnb tells the guests that the stars mean one thing and the host experiences it meaning something completely different with penalties : i.e. less than 5 stars is a fail.
Not sure how Airbnb explain 'Overall', but my guess is it's worded in a nebulous fashion and open to myriad interpretations such as overall vacation: less than 5 stars can mean it rained, they got lost on the metro, they had an argument, they didn't like the color of the sheets even though extremely comfortable. They expected more for less! Or in a response I got "ny is nothing like the "gossip girls"" ding!, ding!. Or an all too frequent rejoinder—everything was fabulous, amazing, wonderful, but nothing is perfect!
As for "What could be improved?" When Airbnb is pressuring the guest to drill down to details via the lengthy review survey it's not hard to find one thing that was never noticed until Airbnb got out the microscope and focused it on minutiae.
"What could be improved and how much more are you wiling to pay for it" would be a far better question.
My guess is Airbnb is collecting data for creating alogrithms and analytics (a gold mine) thus they will never streamline the survey length review or make the rating sytem simple because doing so means less data to amass, play around with, manufacture perception and expand their business.
I have given up on Airbnb doing anything for hosts past offering the site as a fee based booking platform.
I do my best for my guests and make improvements when feasible and that is as far as I am willing to go these days. I am not a mindreader, I can't subsidize guest's vacations, and there is no accounting for people even though technology has made a fortune trying to convince us otherwise when it's really just all about selling.
The review and star system is not designed per se to help hosts it is designed to help Airbnb.