why is my health gain with Wisp so wildly different each game?

I only started playing her yesterday, so I'm still trying to figure some things out. My Strength is around 160 + Molt Augmented and Arcane Bellicose.

Most game I only get to around 1.1k Health with her Reservoir, but some games I get to around 2.8k, and I have no idea what is different each time. I haven't changed my build or anything.