Well, we now have “DUI info for drunk driver”. Possibly an instruction manual.
This thread seems to have completely confused poor Google.
Like posting to a thread about them? 
In broadcast media advertisers can buy ads Run-of-Station (ROS) at a relatively low rate. The ads will be salted in at various times of the day, filling the interstices that the ads at a higher rate for placement at specific times (like the morning drive) leave. I would imagine Google has a similar deal with the addition that someone can pay a bit extra to be the most common default ad.
I think Desert Dog has probably described what’s happening; Google has some sort of default and right now that’s it.