Have you noticed that sometimes the Availability chart for your Web Uptime Test dips below 100%? But you don't have any outages listed?
This means that we have observed a drop in connectivity, which does not match your alerting threshold - such as one node seeing a connectivity blip, and another not when you have a minimum of 2 failing nodes set (Sensitivity on 2).
We still record that the fact that there was (possibly) a drop in connectivity, and so this will reflect on the graphs you are presented. If you would like us to investigate any potential problems, or false positives, please just open a support ticket with as much detail as possible and we will be happy to investigate.