It's not the mirror, it's a monk thing - their weapons mostly sheathe upside-down on the back, due to how they grab them during the Jab animation.
Thanks Stop! Now I know, and I can stop trying to duplicate it!
That is one thing that has always annoyed me about monks, though I do sort of wish we had the option for some staves to be upside on any character!
Maybe they should do it for other classes! I reckon rogues should pull their weapons out from the bottom rather than the top (unlike warriors) as they usually are being sneaky and coming up to stab you in the ribs rather than crack your head open...