Complete from A to Z

Why do things that say they are complete from A to Z, contain things past Z? Like zipper, which is past Z.

But not if you consider the alphabet a big loop and you are looking backwards from A…

I’m not sure if the question is serious, but if it is, the simple answer is it is a figure of speech. It is much easier than saying complete from A to Zyzzyva (which is the last word in my dictionary here, defined as a tropical American weevil destructive to plants.)