[Thorman]

If these graphs were segmented by device, network (search vs. display/youtube) and the country it would tell the story. Users are doing more searches and clicking more as a result of the convenience of mobile devices. Desktop users were and are more intent-driven (worth more to advertisers), but now comprise a much smaller percentage of the total. Display and Youtube impression inventory may be becoming a larger percentage of total, but has lower value than search thereby bringing down the average. As stated by another user international expansion into lower income regions would lead to lower average CPC as well.