I'm not perplexed that kids are going round saying, "6-7."
What perplexes me is all the adults complaining that "it's annoying! I don't understand what it means!"
That. Is. The. Whole. Point.
And, moreover, that's been the point of youth slang since the dawn of time - including when you were young.
If it's any consolation, it'll happen to today's youths too. As soon as they grow up.