Are the injectors allways putting the same amount of fuel in each injection?

 

No. The amount of fuel injected is dependent upon many factors, the most important of which are duration (of pulse) and pressure differential (between the fuel rail and the cylinder). This translates to an "area under the curve" amount, if you recall high school calculus.

Certainly the ECM knows the length of the pulse it sends. But it doesn't know precisely the pressure in the cylinders, and I'm not sure how precisely it knows the fuel pressure. Then there is the variability in the injectors themselves - they don't all react identically to indentical pulses.

Thus the overhead is a good guestimate, but for accurate info you need to do hand-calcs.