Botox

I know that Botox bottles are single use only, and my physician does follow that. But I have a question. He opens a 100 unit vial for me and uses the whole thing (I have my forehead, glabella, periorbital and perioral areas as well as some of my platysmal bands treated): but when he mixes it with saline, he tells me that he gets 400 units out of that one bottle of 100 units. Am I getting 100 or 400 (!!) units of Botox when he treats me with one bottle? I don't understand. Thank you.