They can do one station or the entire cluster. They can also do specific stations within the cluster.

Maybe someone wanted to reach the Urban audience only. Take iHeart for example. They could likely do a package deal. Buy both FMs, get the AMs for free or a reduced price. That is actually common practice.

Should point out, a lot of ads you hear especially from national advertisers are barter. They are just paying for network programming and other things stations use like prep services. So it doesn't matter what station it is, they still have to run the ads.