Phones are the same way. 2 people with the same exact phone will get different battery life.
I might get 4 hours of screen on time from my phone and another might only get 2 1/2.. Far to many variables and everyone uses them differently..
I think there are 2 separate issues here that many people mistakenly conflate.
Certainly, your battery life will vary tremendously depending upon your usage. That's a fact.
But subject to the EXACT SAME usage, a given device should perform to a certain standard and that benchmark can be reproduced by experiment. Otherwise, all the battery tests performed by the many magazines and online reviewers are meaningless.
So this question here is not whether you and I will get the same number of hours of battery life for a given device. The question I'm trying to answer here is...what is the relative battery life of the M3 vs. the i5? So, if my test shows that the M3 gets, say, 10% more battery life than the i5, I think it's reasonable to conclude that you or I will get approximately that same differential based on our particular usage patterns.