There seems to be an issue with the battery indicator on the iPhone 6s and the iPhone 6s Plus, which prevents the battery indicator from reducing proportionately with the actual charge of the battery, thereby suggesting you have more power left than you actually do. Apple is aware of the problem and has suggested a simple fix for it. It seems the issue is caused by the device time settings, so moving between timezones causes the battery meter to malfunction. The fix is to either restart the phone or check if the clock is set to update automatically through the settings. A more permanent fix should be rolled out in the next iOS...
Source: http://www.gsmarena.com/apple_acknowledges_issue_with_iphone_6s_battery_percentage_indicator-news-16131.php
