Your absolutely right about the rule and it would be correct to write CD
and civ which is 100 +5-1 for 104 BUT BA
really DID choose those combinations to represent 400 thats hundred four times !
Anyways this is getting esoteric and not really anything to do with aircraft Thanks AASilverbird.
In the UK we have a habit of using mis-interpretations of letter combinations to represent differing words within car registerations that has passed into the aviation world.
In the case of BA
they also use G-EU-- on their airbus aircraft to indicate they are an EU airline but wouldn't put an EU flag on the aircraft for FEAR of being castigated in the right wing British Tabloid Press !