Or maybe IOS 9 is more efficient on 6s than 6 and your logic doesn't hold up at all, so how about that hey...
While there may be a slight battery improvement from IOS 9, most of the improvements are usually hardware related (CPU/GPU uses less power because it is on new process, COMM uses less power, even memory possibly uses less power). It's even possible, like the original Air, that it could give more battery life than they said it would in most use cases...
Battery life vs functionality is always a tradeoff; a tradeoff most people are ready to make.