actually, gsmarena rating includes standby performance. Good video/web/talk can surely has bad standby. Since most people's phone spend lot of time in standby, this perhaps is the most significant thing gsmsarena considers that nobody else test.
In my judgement, gsmarena is the only one I fully trust.
Of course, Sammy still have time to push out 0 day updates to fix s6's standby performance, which I suspect is bad and respincible for soso overall battery life right now.
Then honestly do you believe S4 would last longer than G3 because they say so? I don't think any G3 owner would agree with that. And this is just one example from their list that seems not in line with reality.