I still think we're missing something. Something else must've happened because since he claims to have grown it because of the scar, why would he wait a year?
You know how pot makes time go by realllllyyy slow? Maybe that's what happened to him. It didn't occur to him to grow a mustache until a year later. (grin)
From The Simpsons' episode where Homer campaigns to legalize marijuana:
HOMER
Good afternoon. I wanna thank you all for coming down here, taking time off from your jobs... (crowd all mutter "Jobs?") Cause we need to keep medical marijuana legal! (crowd cheer) Whether you suffer from glaucoma, or you just rented The Matrix, medical marijuana can make things fabulous! ...medically. (crowd cheer) So my mellow fellow citizens, when you go into that voting booth on Tuesday the 7th...
GUITARIST
Uh, Homer... that was yesterday.
Homer goes to a newspaper-dispenser and buys a newspaper.
HOMER
Oh man, they already voted! And we lost! (crowd groan)
Oh man, I can't believe we spaced on the date! Heh, heh. Hence my theory.