I think it's trying to tell you that conditions are right for
ice.
But, you're right, it seems a little inconsistent when it shows up. I'm not sure what all it figures into the equation (if anything other than temperature).
It could be that there's a window. For me, it seems to turn on right about when the temp falls below 38 degrees. Maybe it turns off again when it's below 18 or whatever, because it figures, at that point, you're in snow (or shouldn't be driving at all
).