There are numerous processes that can vary day to day, night to night on a phone. So variance is not too unusual. However, if you've checked those kinds of things (i.e. more mail one day, larger attachments one day, programs downloading data every few days, etc.), more than likely the variance you're seeing is signal related. When data signals drift out/in and the phone has to work harder at re-acquiring the signals, it does a number on the battery. And this is something you don't usually expect when you're investigating only errant programs/processes. Just something to think about.
Add to that the fact that when you "unplug" your phone at night, even after having a green light, it could be at 100 percent one night at that time but 93 percent at that moment the next night. Just due to the nature of how the charging mechanism works.
Recommend keeping your phone on the charger all night.
-Frank