I hate to be the one to break this to you, but there are other things in a phone besides processor that consumes battery.
I am more than aware of what destroys battery life on cellphones. But what you refuse to take into account is that before any apps are installed or running in the background, before anything is done to the phone with a particular user, the difference between the processors dictates what the end users have to start with.
The more efficient it is will determine just how long it will go between charges, regardless of what the end user does with it. If one draws more from the battery than the other, the one drawing more provided both are being used the same, will drain faster.
I have used both phones extensively and use both in the same manner,understand this.
I am not saying that the 1520 is a slouch in the battery stats by no means, however the m8 is better because of the newer version of the processor with it's optimizations for battery management and the improvement built into the chipsets.
This is not subjective to just my opinion, it is a undisputed fact.
I'm a power user and as being one I opt for the chipset that offers the best power management from the start to fulfill my needs. Newer versions of the same chipsets offer this... and they always have.
Everyone's results will be different because of how they use their devices even with the same device and the only objective comparison that I consider valid is for equal usage on both over a period of time.
I have done this.... Have you?
I'm sorry you don't believe that a newer chipset makes a major difference between these devices, but I am not alone in seeing this here as others have stated.
I do suggest that you actually research this properly by visiting more than 1 site so that you can get a real understanding of what the cpu does for better battery life, after all just having a larger battery doesn't mean that your phone will last longer than one with a smaller battery but newer processor.