The "Miscellaneous" battery section is basically discharge that the operating system cannot account for. For the Nexus 5, that happens to be insane WiFi drain. The Nexus 9 doesn't even list WiFi for me in my battery drain, but if you add up the battery use, it doesn't even get close to 100%.
Unless they changed the way in which battery percentage use is calculated, it stands to reason that the WiFi is drawing all that power.
I'm going to run a test tomorrow hopefully, play 2 hour video from local content and see what the battery drain is, then do the same with WIFi. WiFi will obviously take more, but not to the significance we're seeing at the moment. It's borderline insane. A 6700mah battery should not deplete within 4 hours on the lowest possible brightness setting. When it's sat doing nothing, the Nexus 9 has phenomenal endurance (I saw a 1% drop in 7 hours overnight).