It may be that for some reason the engine is getting hot, but the coolant inside radiator isn't - at least not until you rev it up and force some of the hot coolant from the engine into the radiator where it then contacts the fan sensor.

Just a few things that come to mind.....

Maybe the thermostadt isn't opening as soon or as wide as it should and needs be replaced. Maybe the radiator core is partially clogged and hindering circulation. Maybe the pump impeller is deteriorating and not pushing water very well. Maybe the belt is slipping (doubtful).