I imagine it´s past experience. If you´ve flown a lot in the (sub)tropics a little green or yellow on the screen is just an opportunity to get your steed hosed off. Also, some guys actually look outside (!!) and make a visual, and usually more acurate assesment of the situation. People who have spent all their lives in the far northern hemisphere will gladly land when the braking action is .00, but get nervous at the sight of any puffy cloud.
Likewise some airports will report CB´s on ATIS at first sight of a cottonball, while others (closer to the equator) will not mention anything until they go above FL350.