Yes Poutine, but you are mixing volume and weight.
16 fluid oz of water (volume) weighs about a pound, lb, weight (a pint is a pound the world around).
But mushrooms ain't water. Their density is less than water and there is a lot of air between mushrooms. So a pint of mushrooms will weight significantly less than a lb.
Yes it is confusing. But I don't think we are going to change any time soon.