"Rather than 4.7% more capacity being responsible for a 12.5% increase in stated battery life, efficiency gains in the tested OS and software also likely contri

"Rather than 4.7% more capacity being responsible for a 12.5% increase in stated battery life, efficiency gains in the tested OS and software also likely contributed, since last year’s test was with Mavericks and an older version of Safari. Fortunately, those efficiency gains benefit old Macs, too." Source: http://www.marco.org/2015/05/19/mistake-one http://www.marco.org/2015/05/19/mistake-one (footnote2, at the bottom of the page)