what rhys is seeing is because the temp sensor for the secs monitor is the ecu one in the head, and the fan switch is at the bottom of the rad.

of course the water in the head will be a lot hotter than at the bottom of the radiator, that's why you need such a low setting for the switch. the fan switch is put at the bottom of the radiator as it then only cuts the fans in when there isn't enough air movement over the radiator core tocool it. if you put it in the head, or on the top hose, it'd never cool down to the switch off point.