16's scanner > Everyone else's. 17, 18, Gero & Cell all being biomechanical with a personality must have hindered their ability to accurately read power levels. 17, Gero & Cell all let their ego get in the way at some point.
Haven't read the chapter yet but maybe his King of Hell attacks aren't always AdvCoC? He wasn't aware he had the ability when he was fighting Kaido so maybe he was doing it subconsciously?