Jeremy is correct. Since the device has a greater voltage to work with when stacked, the Sigelei can maintain a constant output much more effectively than if you are in a single battery config. The Sigelei maintains a constant output by increasing the "duty cycle" of the square wave as the battery voltage drops (the square wave is made HIGH for longer periods of time than it is LOW). In other words, it's not the battery voltage that is boosted, but rather the amount of "time" in the square wave that the voltage is applied to the atomizer. As the battery voltage continues to sag, the duty cycle will eventually max out at 100% as the device attempts to maintain the output power you've set. Once that happens, the device can no longer increase the duty cycle to compensate for battery sag since it's maxed at HIGH for the full period of the square wave. You can see when you've reached that point on the V5 by noting the reduced output power displayed on the screen.
I don't vape at high power or use low ohm coils, so a single 18650 battery last a full day (or more) at around 8 watts and 2.5 ohms. Those that prefer to vape at high power levels and/or down around 1.3 ohms will get better performance when using stacked 18350's.