A strange thought just popped into my head: What if Google's content network pricing algorithms (smart pricing, etc.) are at least partially driven by the need to ensure adequate inventory ('supply') of ads to run on publisher sites on any given day?
In other words, in order to prevent showing blank ads or PSAs on lots of sites, they need to drop the actual per-click payout rates across the network in order to ensure that advertiser daily budgets are not exceeded too quickly... in order to have the ads shown across the network throughout the day.
