I'm guessing the difference is due to the OLED screen and lack of dedicated separate backlight, along with a lower capacity battery from having a more power efficient display.
Sorry, I have to always counter this stuff. OLED is *not* power efficient in many scenarios - in fact, it's ONLY a win when it's mostly off (ie, lots of dark pixels). I've no idea how the OLED marketing guys managed to get this one into the collective consciousness, but it's a total lie.
If you're displaying white content (eg web pages), then you'll find OLEDs are up to 2x higher power than an LCM. That likely accounts for why the Zune UI (and actually, a lot of android UIs) use black backgrounds.