On software updates, we can choose country pools or specific servers, when checking servers on https://launchpad.net/ubuntu/+archivemirrors we can see that some are up-to-date, while others are lagging. If we choose a country, will update address only the up-to-date (or less lagging) mirrors ?
If you choose a country, mirror is selected in such a way that it is fast as well as up-to-date. In my opinion up-to-date mirror is given priority. But if two or more mirrors are up-to-date then the fastest mirror among them is chosen.