the definition of heavy use may be the difference. I consider myself a moderate user, and I get about 12 hrs with auto brightness and some widgets and a live wallpaper. Mostly on wifi.
I send email 4 or 5 times, text about 40 or so times, use flashcard buddy and Evernote, and goof off on the Internet. My phone stays on airplane mode from 2 a. m. till 8 a. m., and only looses about 2%.
it's not rare for me to have 24 hrs of battery. I rarely leave it plugged in if it doesn't need it, so I often go to bed with it discharging. So I'll get a few hrs one day, overnight, then most of the next day.