Exclamation Xperia Z1 - Stamina mode only at 5 hours in 100% charge

Ok I just bought an Xperia Z1. Everything seems to work fine except I can not get the Stamina mode to increase to 6 days and more. I've seen people getting it up to 15 - 20 days!

All notifications are off. Basically I turned off everything. Stamina mode @ 100% charge is 5 days & 11 hours. If I turn the Stamina mode off.. Just 2 days and several hours.

Did I get a faulty battery? Any other settings I should know of. I havent downloaded anything from the PlayStore.