First off it's 10 popular sites. To create statistics a person only needs enough data to get an adequate sample. Finally, Google went down the Android road in order to gather enough data to sell advertising to businesses. Unfortunately, very early on Google determined that Apple users were doing the bulk of the browsing... far more then their proportion of handsets. Now, since Apple is moving to marginalize Google apps from Apple users, Google is losing more and more data (that drives their pricing to advertising clients) Google is working hard to make up for the shortfall with such things as "Chrome."
I don't expect that Google will lose quickly nor Apple win quickly, but the above information is crucial to seeing how the struggle for user data is playing out... whether you understand its significants or not.
I don't understand the significance. I think it's because the issue really isn't significant, but I'm willing to concede ignorance of the process if someone can demonstrate how knowing the ratio of operating systems browsing the web either benefits or hurts either Google or Apple. Since you used Google as an example, let's go with them. I present Google with a report that says iOS devices account for roughly twice as much web browsing as Android devices. As a Google executive, what do you do with that information?