A regular syringe will empty a few hundred microliters of liquid when the plunger is pushed. When the plunger reaches the end of the regular syringe there will still be some liquid in the small channel between the plunger and the start of the fitted needle and inside the needle itself. This is called dead space volume and it can be 70 to 90 microliters. After five uses that adds up to about 400 microliters, almost half of a 1 milliliter (1 cc) syringe, or at least one extra sixth dose.
There are, however, syringes like special insulin syringes that come with an attached needle and have a very thin, and very short channel between the plunger and the needle. Some have a thin plunger extension that also pushes into the thin channel. As a result of this, these syringes have a very low dead space volume, from 2 to 5 microliters. So, if you have these low dead space syringes, you do not lose that 400 microliters after five injections, so several hundred microliters remain in the vial to give you one more shot. The six shooter!