Sounds more like Armor reduction... Not a bug... But thats just me.

Rain 1 is 150 x 1.

Rain 2 is 150 x 2.

Rain 3 is 150 x 3.

I've no idea where you got your numbers striker. Anyways, if I had to guess I'd say the tooltips are meant to be the total damage but are just off (i.e. 100x1, 100x2, 100x3), and not suggest 3 waves of 300 damage.

Edit: Upon further checking, it's only the tooltips in the skill tree that are off, after purchasing the ability you get the right ones on the bar (states 150, then 2 waves of 150 each, and 3 waves of 150 each).

Alright, I just played a round as the Torch Bearer using ice.

The number of waves of damage is the same as before.  Level 1 Rain of Ice hits once, 2 twice, and 3 three times.  The damage done by each hit seems to average at around 150 damage though, so the actual damage looks closer to 150>300>450 than the listed progression.