The new SSD is probably where it's coming from. I think Apple is using the Samsung XP951 in the new MBP (I'm not 100% certain though) and I know they have a drastically reduced idle power consumption over the XP941 in the 2014 MBP (2mW vs 50mW). I'm not sure what the difference is in load power consumption though.
Aside from the SSD, there might be a bit of marketing involved (e.g. I can get 10+ hours out of my 2014 MBP if I turn the display brightness down, turn off Bluetooth and don't do anything strenuous). Point being, they probably could have marketed the 2014 MBP as 9 hours and gotten away with it as well.