Usage =/= on screen time
I've only seen the usage time go up when the screen is on. If I let it sit for 20 minutes or hours without using it and come back, the usage time will be the same.
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Usage =/= on screen time
http://www.apple.com/batteries/iphone.html
"Usage: Amount of time iPhone has been awake and in use since the last full charge. The phone is awake when you’re on a call, using email, listening to music, browsing the web, or sending and receiving text messages, or during certain background tasks such as auto-checking email."
I don't have my email auto-checking so it's not doing that when the screen is off.