Really? Or does that just work because it's numbers? I can't imagine how that's useful because without the letters the same 12300 could be in Vancouver or Montreal couldn't it?

I don't know much about credit card validation, but I have the impression that the zipcode is just one more correlation that tells them you're more likely to be the real owner of the card.

If the 3 numbers I type match the 3 digits of the owner's postal code, it's probably a reasonable signal that I'm the owner (even without the letters).