Interesting point but you have to remember you're not really comparing apples to apples here. The link you provided compares a Canon f/4 VS Sigma f/2.8 which isn't a true apple's to apple's comparison. More like a peach and orange.
DXO Lab tests thousands of lenses under Strict controlled environments, I highly doubt they're fluffing up the test results. PLUS "IF" they were...what are they gaining from doing so?
Something to think about.