By default I believe it's in low power mode, which is "instant on" so it'll respond to voice commands even when it's "off".
You can turn that off by going into your settings and changing it from instant on. I believe it's in the power/start up section of settings. --- Pokemon Black 2 FC:0476-9471-9375 Xbox Live GT:Eren x Yeager
I have found, for me at least, overall my xbox runs better if i keep mine in power saving mode. Sure you lose like 20ish seconds of boot up time, and the ability to say "xbox on," but overall i think it is worth it to just keep it in power saving mode. From what i have seen, although i could be wrong, in instant on mode the xbox does a terrible job of clearing out the cache and the UI becomes a little less responsive.
If you ever run into real problems with apps running/errors just hold down the power button, while the xbox is on, for like 10 seconds or so and it will clear it. Plus i treat mine like a PC and close every app when i am done with it instead of leaving it running in the background.