There are actually a few different things at work.
A 3.7V Mod will deliver 4.2V with a fresh set of batteries. There is no regulation circuit, so although the voltage drops off gradually as you use it there is 100% efficiency, as far as the circuit is concerned.
Now, as far as voltage regulator circuits are concerned there are many different types of circuits. To keep things simple I will just mention a couple of things.
There are "buck"-drops input voltage to desired voltage, "boost"-raises input voltage to desired voltage and "buck boost" - can do both. The most efficient of the three is the "buck" circuit. Which is what Notcigs uses. In fact, without getting into other details, suffice it to say Notcigs produces very efficient Variable Voltage PVs.
Efficiency doesn't just effect battery life, although that is a big thing, as we all know.

Inefficient circuits will draw more current than a more efficient circuit to attempt to maintain the same voltage. When the batteries struggle to keep up with the necessary current delivery the Voltage can "sag", as in it drops to lower than what it should be. I suspect this is the main reason you are seeing performance difference in VVPV's.