Are you sure it's solely the amount of traffic leading to higher CPCs? Perhaps your higher traffic articles from December are on topics with a higher CPC? Time of year might have something to do with it as well. It makes sense to me that CPCs for a lot of topics would increase leading up to Christmas.
These were my thoughts exactly. The guy cannot draw conclusions from the traffic and CPC numbers alone. The content's changed, the traffic sources probably changed (the sudden surge had to come from somewhere), the time has changed. I earn money on mobile app's ads. December is by far the most profitable month and I imagine it's purely because people buy more crap before Christmas, which drives up the demand for ad traffic.