Google has figured out how to target advertising to the person on the computer and not only based on the website you are at ... For example, I occasionally go to my hometown paper in Wisconsin and some of the ads on the site (not popups) are for businesses where I now live in Texas. I doubt if those businesses are focusing their on-line ads on small town Wisconsin newspapers.
However, the website owner can request that certain classifications of ads (such as adult oriented sites) not appear on their site.
Or at least that's what I remember from a couple "Google certification" courses in a prior life ...