Background story first.
These sorts of studies are usually based on ad impressions.
When my wife browses on her iPhone 5, she sees ads, because she uses Safari and she hasn't found (basically hasn't looked for) a plugin to block the ads.
When I browse on my Android phone, I don't see ads, because I use Chrome with the AdBlock extension.
I browse on my phone far more than she does (she uses the iPad, which I'm lucky to have any time with!).
The question: do these facts combined mean that, even though my phone browsing is probably 10x hers, our combined usage as tracked by these surveys based on ad impressions, would either only count her usage or make it look like we only use an Apple device?
The broader question: if a higher proportion of Android than Apple users block ads (and the prevalence of blocking extensions for the stock and Chrome browsers suggests that might be true), would that bias these surveys in Apple's favor?
To answer this, all you have to do is look at the tiny fraction of users using Chrome on Android (if is like 3% last I checked a couple of weeks ago) and estimate the fraction of those users install an ad blocker (actually very small) and see this source of error is very small.