droplets is the main difference, small water droplets freeze instantly on impact with the leading edge forming rime ice, whereas larger droplets impact then run back over the wing surface before freezing there as clear ice.
Temperature is more to do with it - approx 1/80 of the drop freezes on impact per degree C below zero (SCWD do not usually exist below -40C), the rest running back, size does come into it though as the colder it is the less likely it is to get large drops.