Occasionally you may see an AP register a very high signal level value, such as +27 dBm, which is very unusual for Wi-Fi. While there are some transmitters that can transmit at that level, they are not common. According to Wikipedia, the typical maximum received signal power in a wireless network is −10 dBm.
Usually, such values are not true-to-life and are most likely caused by an error in the way that particular transmitter communicates with Airport. NetSpot 2.1 fixes this by ignoring unreal values reported by AirPort. You should disregard the readings for signal level, signal-to-noise ratio, and signal-to-interference ratio from that AP.