The inc shuts down shortly after 4% so I don't think it truly ever completely drains the battery. Like was mentioned probably a safety feature. I've used it until it shut off a couple of times and haven't had any problems. According to Battery Left my full battery life is 23:08, and I'd say that's in the ballpark under my typical use.

Re: Battery Left Calibration
 

Yea mine shut down around 4% I still have a few 2 red spots after charging, one around 30% the other around 96% Hopefully it clears up next charge.