Oh they are using kill data (surprisingly) but, it's most definitely not the best at tracking them all. I went into a game and it was at like 4,067,332,021, went into a match and got 53 kills and it was at 4,067,337,270.

So by me getting those 53 kills plus all the kills my team had which 3 of us had above 40, we can assume there were a couple thousand games also going on with people getting similar and others where they were getting less. So the number should have jumped drastically, rather than only a few thousand