If i knew why some people's battery are draining faster i would be working at apple and a fix would be on it's way.
All i can say is i've never had a problem with battery life. I rarely let it drain completely, normally top it up when it falls to 50-70%.
Yesterday i did my once monthly calibration on the iPhone and iPad. The iPhone 4s had lasted 3 days. That was
1day 16hours standby
8hours 12mins of usage
Which is frankly amazing. That included, lots of browsing, twitter, lots of music, email, phone calls, sms, photo messages and a small amount of other app usage.
Interesting to note, the iPad takes twice as long to charge with a higher wattage charger, so it must have at least double the capacity. Not surprising considering the size.