I'd be more than willing to bet that they controlled for these variables you mention, else the test would be useless. Sure, it's right to be skeptical, but they do this for a living.
Given Google's stake in having accurate data, they probably bothered to do it correctly.
(And I am not usually a defender of Google.)
I guess that means they controlled for the other variables.
If that is what is happening, then I can see why Google would lose. That is, their main conclusion would be that people tend to check organic results first. If that's true then if a company that has to pay to be page 1 is organically moved there by increasing the number of results displayed, then G loses.
They check this by seeing if revenue goes up when they limit the results to 5 ads.
If you're going to come to some grand "Aha!" moment, you should define your experiment. Were these the same users? on the same day? searching for the same stuff? What browsers were they using? Did they have their windows maximized? What was their screen size? ... and so on.