For what it's worth here is my theory on this.
With a Cartomizer your juice is all soaked into the filler. So as the amount of juice in the carto gets lower its still soaked into all the filler. This means that less is getting to the coil and the amount of vapor produced starts to drop off so less juice is used per drag.
With the CE3 the liquid gets strait to the tiny little wick. This means that there is always a good amount of liquid at the coil during firing, Allowing each hit access to the same amount of liquid. This means no drop off on the amount of vapor produced leading to fewer drags of more vapor as the amount of liquid starts to get low.
I don't think I explained that very well so I hope you get the idea.