And to note, JENOVA created nothing inside Tifa's mind. The only reason Tifa isnt telling the truth as she knows it is because (SPOILER)she fears shattering Cloud's very fragile mind. If he's straight-out told that everything he believes is a lie, it's very possible that he'll break down. She knows this, and thus she very gently 'tests' his memory ("Do you remember the promise?") seeing how much of the real Cloud is still there, and very slowly trying to correct his memories.