Who's to say GSMarena didn't do a battery test on the pre-production model of the S5 last year as well. Then you would be comparing apples to apples. Seems like everyone is trying to make excuses for Samsung here.
No they are comparing pre-production S6 to latest S5 test result after Lollipop update. They bumped up the S5 battery rating with Lollipop update recently. So it's not really apple to apple.