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.
I've found battery life to be pretty darn good, and if you've seen shots of the Zune/Window 7 interface, you can see how heavy it is on the black pixels

Perhaps that's the reason?
In my opinion, the Zune was dead on arrival when the featured color was brown. And not a nice chocolate brown, either. More like '70s rec-room fake wood paneling brown.
I'll just point out that the Zune HD was a pretty different device.