This is because the early VV PWM(pulse width modulation) APV's were set to Vavg and not Vrms. To get the true output voltage on a PWM device you must use Vrms.
Which is odd, I've heard that analog multimeters are good at reading PWM because they even out the voltage to give you a steady reading.
In RMS mode a Zmax and Vamo, for instance, will put out a whole volt lower than the set reading. In AVG mode it puts out exactly what it's set at.
Now, either the information I heard (Analog meters reading PWM) was wrong, or it's simply an issue of the (relatively) long full voltage spikes of low frequency regulation causing a hotter vape than the readings would suggest. Many people including me can detect a difference between flat-output signal devices and the choppy signal of the 33hz chip used in most VW china mods. The coil is unable to "average out" the voltage, which causes high temp spikes in the coil, and a vape that is hotter than it should be. This is why you hear the rattlesnake sound, you can actually hear the coil turning off, and turning on very quickly. If it's enough of a difference to hear, it's definitely enough of an issue to cause large temperature variation.
Trying to get accurate output readings isn't as important as getting accurate heat and the coil. If you are using straight 4.2v straight from a battery, and measure the temperature of the coil, it should match to what the temperature of the coil reads with a 4.2v setting on a VV device, taking into account voltage sag of course. While I'm unable to test actual coil temps, from personal experience, and the words of others with the exact same person experience, I'm convinced the low frequency regulation causes a less desirable vape, the extent of which depends on what specific heads you are using.
Anyway, for the OP's question:
When talking cartos and the like, usually higher resistance ones have a longer coil, meaning more surface area. LR stuff usually has fewer wraps of the coil, which gives you less surface area.
It's very possible to build two coils that are the exact same resistance, and one is twice the surface are of the other. Thicker wire reduces resistance, longer wire increases it. So if you made a very long, but very thick coil it could match the resistance of a shorter coil with less surface area.
But generally manufacturers use a longer coil with higher resistance stuff. With a longer coil, meaning more surface area, you can pump more power into the coil without it becoming too hot.
if you have 1sq in. of surface area and put 10 watts into it, and have 3sq in of surface area and put 10 watts into it too, the 1sq in. surface area coil will get hotter than the other one, but it will also have less surface area, so less potential for vapor production.
If you have a device capable of putting out a lot of power, higher resistance stuff will (generally) put out more vapor at the same high-power setting compared to LR stuff. But if all you are using is eGos or cigalikes, or fixed voltage mods, you aren't providing enough voltage to take advantage of that increased surface area, so performance suffers.
It totally depends on the coil design, but generally this holds true.
