
Other mistake: When Gomez begins to flip through his desk calendar, we see that "today" is Thursday, March 28, 1991. He then turns to the date of the "next quarter". The left hand page shows Thursday, August 8, but the right hand page shows Tuesday, September 9. (00:11:15)
Answer: Presumably when Fester was lost in the Bermuda Triangle, whatever supernatural forces caused him to lose his memory also made him grow hair. This gets reversed at the end of the movie when he's zapped by lightning, restoring his memories and destroying his hair follicles as well.