Not sure how anyone could make that assumption, it's based on what test, specifications, notation by Apple...doubt that.
https://www.forbes.com/sites/gordon...one-8-plus-whats-the-difference/#12701d1d6ad7
In terms of pure stamina, iPhones continue to stagnant and the
iPhone 8 (1821 mAh) and
iPhone 8 Plus (2675 mAh) actually have smaller batteries than the iPhone 7 (1960 mAh) and iPhone 7 Plus (2900 mAh). The
iPhone X has a 2716 mAh battery, but its battery life falls closer to the iPhone 8 than iPhone 8 Plus - aka not great.
By the 1st to specs above, Xs Max gets 2hrs more than an X, for the above to be true, ip8+ must get more than 2hrs greater than the X. Andriod are going well over 3Ah batteries, near 4aH now...lotta energy if they catch fire

. 4GB ram...uh, typical Apple, they had accesss to Samsung's 8GB ram, as some Andriod now have, and I'm guessing the 2019 update will also skim on the newer/faster 8GB LPDDR5 spec...2x the bandwidth of LPDDR4 used in the ip8(faster LPDDR4X in ip8+ & X).
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