I think it goes back to the old joke about the guy on his first day in prison (or at the fire house, or the factory, or wherever). They're all together and one guy yells "42" and everybody laughs. Then another guy yells "25" and everybody laughs. The guy finally asks someone sitting next to him what's going on, and he explains that they've all been together so long they all know the same jokes, so they just assigned them all numbers. Now instead of telling the whole joke, they just shout out the number. The guy figures he'll try it, and yells "63"... but nobody laughs. He' s embarrassed, and asks the man next to him why no one laughed. "Isn't there a joke number 63?" "There is," the man says, "but you didn't tell it right."