The display-on hours that the iPhone displays refer to the last 24 hours and also take into account the hours you use it while it's charging.
To properly monitor battery life, try unplugging it at midnight and using it until midnight the next day without charging it.
At that point, you'll have the actual display-on hours.
To properly monitor battery life, try unplugging it at midnight and using it until midnight the next day without charging it.