Has wi-fi been on the whole time? Or have you had it off, and the battery stats show that it's been on? I've seen one or two mentions of the latter issue.
See this kind of statistics on my phone as well, in spite of Wi-Fi is off.
But battery life is surprisingly good and I think it's a (another) flaw in the battery report. It also has a bar showing Wi-Fi always ON, which is not true.