Bono hates saying "Catorce!". It's too difficult for him.
He said so himself, in an interview before a gig in Spain.
He even asked the (Spanish) interviewer for an easier number,
so the guy said "Quince!" (fifteen). Bono said: "Ok, I'll sing that tonight",
and thus it was, that night in Spain: "Uno, Dos, Tres, Quince!".
The guy was there, broadcasting the concert, and couldn't believe
that Bono had remembered, and changed Vertigo's intro only for him.
As for the meaning of those numbers, I think the "sudden drop" perspective,
and the "alcohol excess" perspective are very interesting. But if you consider U2's
lyrics as a whole, and specially the lyrics on HTDAAB ("...all because of you, I AM..."),
it becomes clear that the "Bible reference" (to the place where God first says His name),
is the best explanation. And a genius subliminal message, too.