Hey, Dan. That's an interesting finding that I haven't noticed, though I rarely use CC these days.
You might check out some of Hobbit's work that he has described and linked on CleanMPG. He seems convinced that as CC tries to maintain speed it becomes too aggressive for optimal fuel economy, with wide swings of RPM deep into inefficient ranges, low and high. (It is based on my experience, validated by his findings, that I usually avoid CC.) His most
recent paper describes what he calls a "forbidden zone," where a certain combination of mid-range vehicle and engine speeds puts the HSD into an inefficient mode. Your results could be from this same phenomenon, where bumping up to 50 from 47 nudges the vehicle speed and RPM together just out of this zone. The miles per hour/RPM chart in his paper would seem to support this.
I'm guessing based on your findings and where you live that most of your CC driving is on level terrain. Am I correct? I would wager that your mileage would be noticeably worse with a CC-regulated 47 miles per hour on hills.