We once had a guest give us a "3" In cleanliness. Since we go to great lengths to keep the villa as spotless as possible, and always receive 5 in that category, we asked him why.
It turns out that he found a tiny, nearly invisible spider web under one of the dining chairs. He remarked that it's particularly upsetting because he "had children in the house". My first thought was that I can't imagine how sheltered a life this family must be living. I'm glad he apparently hadn't discovered the various wildlife living in the area. That could have triggered a nervous breakdown.
The point is, that despite your best efforts, there will always be some who discover something they feel is objectionable. Over the long haul, getting a straight 5 star rating is probably impossible.
Unfortunately, even one guest's choice to click a lesser rating in even one category can negatively impact your overall rating for years, and probably forever.