Bigger screen and ambient display probably the culprit here. Ambient display being on takes the Nexus 6 from 330 hours of standby to 250 hours according to an article I read. Huge consumption for a breathing notification.
Don't take that chart as gospel. I had the Note 4 and I'm using a 6+ right now, and the 6+ toasts the Note 4 in battery life, but it scored way lower in their test.
Honestly, if it got 6 hours on screen time I would be MORE than happy.
My 1.5 year old Galaxy S4 gets maybe 1.5 hours on screen time (with a lot of features turned off, Google now, etc). Maybe, maaaaaybe 2 hours if I turn off everything.