Also, adverts change every time a user views the page, even if there is no change in the wording.

It all depends on companies budgets, and sometimes they decide only to have a certain number of clicks per day/week/month.  Once their quota is done, they no longer advertise, and other companies fill their shoes.

Also adverts get rotated, so that other companies get a chance to advertise and pay google for advertising.  I've been on pages on this site (and others that take google) and although there's no new content, the adverts change, even minutes afterwards.

And as google want to maximise revenue, they might want a user who visited recently, but never clicked on any adverts, to view a different set of adverts to increase the chances of clicking.