I mean literally, it is an actual joke. In this case, it might not have been, but it has been told as a joke for a long time. (The most popular variation is "Why amen and not awomen? Because it comes at the end of a hymn, not a her.")
I'm hearing a lot of "but they always make the 'it was just a joke excuse'", but I think it is kind of relevant that in this case, it is factually true.