If the Mayor is a Werewolf, they don't want the players to guess the Word; their winning condition is the same as any other Werewolf. In this case, the Mayor might want to choose a slightly harder Magic Word. During the day phase, the Werewolf-Mayor may mislead or even lie to the players when giving tokens to lead them away from the correct answer.
Remember though, when voting begins the Magic Word will be revealed, so players could catch the Mayor out if they lied too obviously about an answer! If you picked a harder word for the players to guess, you should reassure them that it was the least difficult option you had.