A is the first letter in the alphabet and so Waterhouse, who has been doing this cipher stuff for much too long, thinks of it as being synonymous with the number 1. In the same way, T is equivalent to 19 if you are working in a J-less alphabet. Add 1 to 19 and you get 20, which is the letter U. So, in the first column beneath T and A, Waterhouse writes a U.