They were hand punched into the die so they end up all over the place. That one certainly is a bit wonky though.
Here is an older CT thread that discusses mint marks. May be worth a read. https://www.cointalk.com/threads/hand-punched-mint-marks.341830/
Yep, the mintmark was hand-punched into the working die until 1990. The employee who punched this mintmark into the die must have come to work with a serious hangover. ~ Chris
If you haven't already been introduced to error-ref.com, check it out. Here is their entry on tilted MM http://www.error-ref.com/tilted-mint-mark/