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.
Since pay per click (PPC) advertisers set a maximum cost per click, and daily max budget, at any given time, there is a fixed maximum payout that must be spread across all available content network web sites. I suppose this could also apply to cost per impression (CPM) ads.
Even if that's not the way it was designed, what if that's the actual result of all those algorithms ticking away silently deep within the Google Matrix... what if they're really, after all, trying to do a form of 'load balancing' - balancing the interlocked and mutually dependent supply and demand of advertisers and publishers?