Immunity vs chance to avoid being shocked

I am following a build guide, and the creator had chance to avoid being shocked on flask (with mageblood), boots, etc. I have two questions on this.

1. Why not just roll immunity to shock on the flask instead of chance to avoid? Are they not the same thing?

2. With the increased effect on the flask, that applies to the rolled stats too in addition to the implicits, correct? Just wondering why he still has the avoid shock on the boots if he can attain 100% with the flask alone.