A complete observation from a non-technical person. I have only two devices... the istick and a Kamry X6 (which is an ego style VV battery, 1600mah). I actually vape a lot longer on the X6, 1600mah battery. The display on the istick must be a significant battery drain? Not complaining, as I can generally get a full day of vaping from the istick......just thought I'd get longer battery time than the 1600mah battery. Oh well, I'm generally pretty happy with it.
The battery capacity is usually measured to the lowest voltage the battery can go (usually 2.6v-2.75v). The Istick cuts off at 3.3v so while the battery might be a 2200mah rating, it's not using the full capacity (and it's not linear). The upside to this is that the battery will last much longer than if you drained down to 2.75v.