I'm guessing, because each encoding swaps one character with another distinct character in a loop that includes all characters, by encoding 66 times you cycle through all characters in the loop and return to the original character. Doing it 65 times stops one short of that and gives the same result as -1 time.

Thats cheating man xd

why does this work?

Not the most efficient solution, but I wish I'd thought of it :)

I cant believe it works.