Those numbers are with hardware decoding, aren't they. I'd be more intrested in something compute-intensive that was compiled to asm.js. I bet they'll last much less.
That said, these are machines with mobile processors and an OS that is tailored for mobiles. It isn't all that suprising that they are more capable than their normal counterparts.
Decoding is hardware accelerated, yes.
Chromebooks seem to average around 8 hours of continuous web browsing[1].
[1] http://www.laptopmag.com/reviews/laptops/samsung-chromebook-...
You are right - it's probably more like 8 hours. The older Acer c720 got between 6 and 7 hours streaming Netflix[1]. The Chromebook 13 got 10 hours[2].
[1] http://www.trustedreviews.com/acer-c720-chromebook-review-ba...
[2] http://www.engadget.com/2014/09/23/acer-chromebook-13-review...