Music language can be a little quirky, as it uses a great deal of terminology that doesn't always make immediate sense. However, it is there for a reason. Adding the second capital letter designates the tuba as an instrument that plays in a lower register, one octave lower than a baritone horn or euphonium. However, since no one actually calls either of those instruments a "Bb tuba" or a "C tuba," when you see "Bb tuba" or "C tuba," you can be pretty confident they are actually talking about a BBb or CC tuba but have dropped a B or a C for the sake of brevity.