I suggest the testers read:
http://www.windowscentral.com/72-hours-new-dell-xps-13 for:
"Battery life is much better without running Google Chrome. We were never expecting the i7 model to get close to the lower spec models in this regard, but running Google Chrome took hours (plural) from it. Without it, using IE 11, I've been hovering around the 7 hour mark off charger with a fair bit of music playing and Photoshop usage thrown in there. Honestly, I'm happy enough with 7 hours. Compared to the 4 or less I've had from a previous, fruity laptop, it's great."
and http://www.anandtech.com/show/8983/dell-xps-13-review/5 for:
"One more note about the Dell displays though is that they have an auto-brightness adjustment with no way to disable it. It seems to manifest itself the most, and the most severe, when there are large contrast changes on-screen. For this reason, our automated testing for the color accuracy would not work at all because of the abrupt changes on the screen as it flashed among the various colors. This would cause the screen to go bright and dim and caused some very erroneous readings."
This could explain the worse battery and lower brightness...