All leaves have a wax layer which is water repellent, but some have an extra layer of powdery wax, which is in the form of loads of little pillars.
Water droplets land on these pillars (not on the leaf itself) and form beads which roll away from the leaf leaving it dry.
Other leaves without this extra layer get the water droplets directly on them which can damagethe leaves in the end.
Experiment with other leaves to see which stay dry and which don't!