Don't know for sure, but my 6s was purchased new shortly after launch in 2015 and the original battery seemed to degrade rather quickly, fortunately I got a free replacement from Apple in December 2016 under the 6s battery replacement program and my new battery seems to be degrading at a much more normal speed. I get the impression that their may have been some QC issues with batteries during the initial launch of the 6s series, but I don't know if they have switched manufacturers or just improved their QC practices. That said I've sure they've had to ramp up battery production for older series of iPhones to meet the demand of the discount battery replacement program this year so hopefully the quality hasn't slid in order to meet the demand!