With the CyanogenMOD argument stated by another member, it shows the same issues as the GPE argument. Maybe drivers aren't the cause, it could very well just be cheaper cells in the batteries. It makes the most sense. How else could Google produce a phone that they can sell for almost $300 less than a [seemingly] identical phone?
The cost cutting makes the most sense. But maybe Google learned their lesson and didn't skimo on stuff, hence the $50 extra tacked onto the price for the baseline N4 vs N5. The cost of memory chips has gone down and I would imagine that the S4 Pro price vs S800 price for the CPU to have stayed relatively the same (like the new iPad price stays the same as the one it replaces)
The technology used in the these products should theoretically be cheaper, plus plastic is cheaper than glass, so that should also cut some cost off of the BOM of the Nexus 5. So basically we are going to end up paying $50 more than we did last year for an entry level device, that might mean better parts.
While the Nexus 5 has less mah than the battery in the G2, the cells could be higher quality, meaning they last longer.
I have a cheap 1600 mah battery for my Airsoft gun and a quality 1200 mah battery for that same gun. You would think the 1600 would last longer, but the 1200 actually does. Why? The battery is of a higher quality