How did you control for the specific ads the browsers were served? Ads are highly dynamic. Unless you (can) include the differences in your methodology you better eliminate that influence.
Why is a different question and it's not that relevant, I often don't care why something I USE works better I just care that it does.
If it something I BUILD then I would care much more but again this is a whole different issue.
The case for more normalized tests is to find out which browser is factually better designed/written.
I don't see anything that would somehow create a bias in favor of a specific browser as far as ad networks goes, if anything the stigma/stereotyping of IE/Edge users would probably mean that ad networks are more incentivized of sending the baity apps towards those browsers.
As for the network part well again that's an important metric if certain browsers perform better at adverse network conditions it's an important factor to know, you do not want to give them the best case scenario every time.
Giving a page a fixed amount of seconds to load is also completely the wrong approach you want to see how browsers behave when they can't load a page properly or when it takes more time than usual, maybe some browsers expend more resources by resubmitting the entire request, maybe some browsers do not parse the DOM tree from scratch when some of the requests stall, maybe some browsers have less resource intensive placeholders for DOM elements, maybe some browsers are better at adjusting the DOM preprocessor for network congestion than others.
So no I can't really see how would your approach would be any better, the approach that MSFT took was quite good, netflix, wikipedia, youtube, facebook etc. with what seems to be realistic user behaviour. What you want to do is to put in test that would produce fair results for fairness sakes that's not how you evaluate anything because it would not yield you any real world data.
What you need here is a big enough sample.