I’ve heard it said that Google may be doing something smarter than just advertising on the basis of the text of the page in which the ads appear. If so, I’ve not yet seen the evidence of this.
For example, here is the title of an article on abuses by the Department of Homeland Security:
And here is the ad Google served with this article:
I’d love to see the conversion rate on that one. This is the reason we employ a somewhat smarter approach with Thoof.