Various anecdotes and Geekbench tests posted online suggested that the gap could be as much as two hours, but an Apple spokesperson told TechCrunch that the variance is small.
"Our testing and customer data show the actual battery life of the iPhone 6s and iPhone 6s Plus, even taking into account variable component differences, vary within just 2-3% of each other," the person said.
The representative blasted "certain manufactured lab tests" as "unrealistic," since they force processors to run at high CPU usage until a battery is drained.
"It's a misleading way to measure real-world battery life," Apple suggested.
Accounts have generally pointed to TSMC-based iPhones lasting longer. The supplier uses a 16-nanometer FinFET process to build its version of the A9, whereas Samsung is believed to use 14-nanometer technology. Energy efficiency normally increases as an inverse function of semiconductor fabrication size.
Apple's dependence on two different A9 manufacturers is likely a result of needing enough chips to meet demand. The strategy may also be a way of forcing price competition, and having a fallback in case of production problems.