I've found the same thing. I think not accounting for where the needle screws on to the syringe explains part of it. When you draw 1mL you inject 1mL, but there still seems to be about 0.1mL you can't push out. 10x0.1mL = 1mL. Maybe the vial is only filled to 9mL, but given wastage you only get 8mL out of it.