Exposure increases this value just by playing a numbers game; therefore, the larger the ad network, the more they can charge.
Targeting increases it more, by allowing companies to choose demographics more likely to be interested in their product, so again, they can charge more.
An interesting aside: While working as a mobile developer, I learned that many apps do not have an in-app-purchase to eliminate ads because it tanks the value of the in-app advertising. Why? Because the people most likely to drop $5 or $10 to not look at ads? They're the ones with the most expendable income, and therefore, users more likely to impulse purchase products advertised.
In other words: By allowing people willing to spend money to support the app, even if there is NO OTHER EVIDENCE they'd be interested in a product, decreases the value of your advertising.
Maybe its because the data that can be collected about safari users isn't worth as much.