I'm so glad to hear we aren't the only ones that have this frustration with the ratings system. We're new hosts who have been working hard to make every stay a 5-star experience. We too had a recent guest rate us with 5 stars in all of the subcategories, only to then rate us 4-star overall. I could go back and ask, but what good would it do since they wouldn't be able to update their review? For all I know, he tapped/clicked the wrong spot on the screen, dropping our overall rating so far from 5.0 to 4.9.
I don't understand why Airbnb has the subcategories if they are meaningless to the overall review rating. The system would be much more functional if the overall rating were calculated as an average of the ratings given in the subcategories. So, in the case I described above, the overall rating would have been 5.0. Let's say our guest decided the location wasn't what they thought it would be, so they decided to give us a 4 star review in that subcategory, then the overall rating would drop to 4.8. The overall rating could update in real-time on the site (or in the app) and allow the guests to adjust things based on their experience. It doesn't do any good for us as hosts to see that we excelled in every category Airbnb deems important, yet didn't achieve the highest overall rating possible -- especially when the guests gave us great feedback and said they will definitely be staying with us again when they are back in the area.
In the grand scheme of things, not a huge issue unless it continues to happen, but frustrating nonetheless.