The time left on battery seem to only consider one of the batteries. Powertop estimate the double time left on battery. I have 2 batteries in my Thinkpad - a removable and an internal. Actually - I see now that kinfocenter shows a consumption rate of 17 W versus powertop saying 8 W. It could also be this that is influencing the estimates...
Seems related to https://bugs.kde.org/show_bug.cgi?id=412139