Is there some joke I am missing here?

Yep. That's really what it says.

Someone doesn't know M from N, and they're VERY confident that they DO,
-OR-
they think they're being clever.

Either way, I find it hilarious and had to take a picture. Because that's what I do. I wonder what that customer thought as I pulled into the parking lot, took the picture, and drove away while laughing hysterically.

I'm such a jerk.