It's unlikely that the battery would drain that much while the whole phone is powered off. The only way you can do that is to remove the battery or press and hold the power button at the top for 3 seconds and hit ok when the icon comes up.

Check the battery, if it is swollen, that could be one reason, unlikely tho. Anytime a battery has a contact to a source, even if it is powered off, it's draining the battery. But this drain is very very very low.

Also are you using Dashwire or another program that starts up when the phone is powered on. That meter is more like one of those predicting gas mileage meters. It recognizes how much battery life you have vs the usuage of the phone. If you are powering it on and have 5 or more apps running, of course it's going to read low.