Apple is really clear about the battery times on all of the currently shipping devices, and compares them to the older 7. iFixit Has also confirmed the battery size as Apple claims during their respective tear downs. As with anything battery related, YMMV, so it would be hard to say that the X is any better than the plus given a single use case.
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Even if done with a series of phones, there are other factors such as cell signal, wifi signal, brightness used, motion tracking, etc that impact how much battery is impacted. Even if the screen is off all day, a person that is highly active and moving around will use more battery than someone who leaves theirs sitting on a desk. Motion tracking and switching towers for signals, and joining wifi all sucks down a little battery life.
Your screenshot is equally vague, and we could quite honestly accuse you of the same type of usage.
If I cared / wanted to argue about it, I could also play devil’s advocate and claim that your screenshot somewhat disingenuous as well. IOS 8 (which the 6+ shipped with) had the ability to show you applications that were using battery life, which is cropped out of your image. You also cropped out the section that would have shown “iPhone has been plugged in since the last full charge” as well.
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After plugged in.
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Cropped like your example
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At the end of the day, I don’t really care if it is genuine or not, as I don’t have a 6+, and my use doesn’t likely exactly match anyone else’s in this thread.