Living in Japan you become accustomed to what's called "katakana English." That's where they've tried to wrestle foreign words into something they can write in the katakana script. A lot of times that means extra vowels are tacked onto the ends of words. My students like to tell me that they "go to beddo atto six o'clocki."
The same thing happens to Kit Kat. If you go with the singular Kit Kat it translates to "kitto katto" which can be translated to "You will surely miss the cut." You certainly wouldn't want to give anyone a single Kit Kat for fear of giving offense.
However, if you go with the plural , Kit Kats, becomes "kitto katsu" which can be translated to "You will surely win!" It helps explain some of their popularity here in the land of the rising sun. In fact, parents are known to give their kids Kit Kats on exam days as a sort of good luck charm.
Although none of this explains flavors like the Soy Sauce Kit Kat or Grilled Corn Kit Kat.