I haven't paid too much attention today, but what I did see with the iPad Pro was that they're using an OLED display (maybe even some kind of double layer OLED for increased brightness if I'm understanding the marketing jargon?).

I believe that OLED is much more power hungry than the previous display type (LED backlit LCD of some type?). I could be wrong, but in TV land that's the case...

Could explain, at least partly, why run time isn't greatly increased.