Two main reasons As already said efficiency . You actually need to be quite smart to choose the right components in order to deliver high power and use the battery to its maximum potential . With the cheap box mods these days , there has to be made some tradeoff in order to keep the price low . Safety wise this is completely okay . Another reason is the so called cut off voltage . LiIon batteries can generally be discharged until they reach a voltage of around 3 . 2 V . Some mods use 3 . 5 V as a cut off voltage , some mods use 3 . 2 V like the alien , thus many people report they get more battery life from their alien or any othert boxmod with a cutoff voltage of 3 . 2 V . These two reasons also determine each other a bit . With a very efficient mod its more likely that you can really use your batteries until they reach 3 . 2 V , because you can still use the battery at this voltage level . Another reason could be that you are just using different batteries , but i guess you are not D