Open a conversation about overfill in a room of injectors and watch it split. Half treat the surplus in a vial as a happy accident they never think about; the other half have quietly built their entire product-cost advantage on capturing it. The difference between those two groups isn't clinical skill — it's whether they understand that the extra product is already in the vial and already paid for, and the only open question is whether their technique lets them bill it.
Manufacturers fill vials so that a clinician can reliably withdraw the labeled amount. In practice that means a small surplus is frequently present. That surplus is not a promise, and it is not free units the manufacturer owes you — it's a byproduct of fill tolerances. But it is real product sitting in a vial you've already purchased, and across a year of vials it is a meaningful number.