It's not the ideal metric, but it's one that is realistic to compute with limited time and resources. To analyze that statistic using a growth metric like "largest 30 day average contributor gain" across such a gigantic dataset, consisting of hundreds of millions of rows and tens of millions of unique repositories, requires resources and engineer time which few people possess. We certainly don't want to divert too much of either of those things away from development of CodeCombat just for a fun data science experiment!
Popcorn Time is awesome, and if it continues (unlike the other Popcorn Time repository) it will eventually pass CodeCombat. However, it might also experience some event or plateau, like Pullup did, that will stop its growth.
A really interesting way to analyze the GitHub Timeline would be to rank repositories based on sustained average growth rate. We chose a fast, (sort of) cheap, and computationally easy way to run these statistics, but there are a myriad of other ways to process the same dataset, which we'd definitely be interested in seeing.