I am not sure just how the iPhone measures 'usage'. If it means when any app that YOU installed is running, then the usage might be correct, should you leave one active that actually consumes CPU (or worse GPS) power when the screen is off. Note that the iPhone is not 'off' unless you do the power off bit. Otherwise, it is in what they call 'sleep mode'. When in sleep mode, we KNOW it checks for mail, checks to see if alarms are due, also for notifications, and reminders, and checks to see if text messages have come in. In short, the only thing it doesn't do is keep the screen on. It's a small computer, and it is never really idle.
A word about the cell network. If you live in an area where tall buildings may affect your signal, the phone may be ramping up the power on your transmitter trying to maintain an active 3G connection. This can cause pretty unpredictable power usage. When I travel to areas where there are few ATT towers, my power usage goes way up.
My phone has Photostream and Find my iPhone turned on in the cloud, email checking is manual. Location apps are active. My average use is about 20% battery power for each day. I recharge once a day.