hi wof,
this is because air at low temperatures (i.e. below -40C) can only absorb a certain mass of water, which is determined by maximal humidity (saturation concentration). The actual value of maximal humidity depends on SAT; the lower the temperature, the less water can be absorbed.
This is basically why there is no risk of icing at SAT below a certain value, in this case -40C (which is in fact defined by the manufacturer).
I hope this clears things up a bit.