Here's my guess. The early Honda bikes in the US were the "C" series, such as the C100 Cub. A "CA" series of bikes followed before the CB superbikes. Thus, to misquote Nigel, "The CB is one higher than the CA, isn't it?"
My guess as to why Honda started with the "C" series in the US is that they had already gone 2 higher than the "A" series they started with in Japan.