I'm not an engineer, nor an expert on batteries and atomizers, but here is my theory:
The e-cig is essentially three parts: battery, atomizer and cartridge. While different makes may use the same atomizer, that is 1/3 of the story. The cartridge is not going to be much of a factor, other than the wicking quality of the fill. If the user direct drips, it's not going to matter at all. That leaves the battery. At the same voltage, there will still be performance differences. Not all batteries of the same voltage maintain a consistent flow of power throughout the duration of the charge. For instance, I noticed immediately that the 510 mega battery seems to perform consistently from start to end of the charge. While the regular battery seems to fall off quite a bit as the charge falls, in terms of vapor production.
My observations on the 510 are purely anecdotal, and may be my own perception. But from other applications, such as flashlights, I know there is a huge difference in performance from brand to brand, even when the same basic characteristics of the battery remains the same.