Those are my favorite passages too. For short-term stay, a perfect rating with lots of review is almost a guarantee that you'll have a great experience.
From my own experience, here's a few reasons:
A high number of ratings is more likely an indicator that the place has been on the market for a long time rather than being an especially great place to stay.
The nationality/culture of the guest can have a huge influence of how they rate a place. In Europe, for instance, a guest from Southern Italy/Spain may not be particularly bothered by noise from neighbours, barking dogs, etc. Conversely, those coming from Northern Europe are likely to be the exact opposite.
Some hosts are incredibly pro-active in garnering ratings (to the point of constant harassment). Others less so.
Some hosts will not accept groups of people, parties, short stays or stays from people with no reputation. As a consequence, they are going to get far less reviews than a less careful host.
Hope that helps?