Answer
The signal strength bar is based on the number of expected beacons vs. the number of received beacons from the AP. APs send beacons at a very specific interval. We use that interval beacon period and DTIM to calculate the expected number of beacons and how many we actually receive. The signal strength is measured in a percentage number (% of retained beacons). This helps us determine what the RF environment is like. If the client is missing beacons, we can determine that the RF environment is noisy. If we do not miss beacons, we can determine that the RF environment is clean.