Not sure what equipment you use. But if you were to dial up just 1 or 2 units on a flex pen, you can depress the plunger even when the needle is blocked. So you would think you had injected when in fact you hadn't. I've had this once since I started injecting.
Also, if there is air in the front of the needle, you are not injecting the full dose, you might be half a unit or so under which makes a difference to some people.