Units are a measure of action

Potency is assigned by a biological assay, so a Unit means an amount of effect rather than an amount of protein. Measured head to head, products differ in how much neurotoxin protein they carry per Unit and how much of that protein is inactive, with one European product carrying 416 picograms per vial at the highest activity per picogram and several Asian preparations carrying more protein for the same labelled potency. More inactive protein going in is what the immune system has to look at, which matters over years of repeat sessions. This is the technical reason a dose is never simply copied between brands.