Firstly, this could be a BD exception which is not honoured by other *A partners. But the other reason could be some some feeder flights on BD only have Y class and you connect to a C class longhaul flight. In this case it would make sense to allow more luggage.

In our case there was not C award space on the first sector (but the flight sold C class seats). But I don't think this was their reason for denying the addl luggage. It was simply that our first sector was in Y and that's when you check-in your bags. They even told me that I could check-in extra luggage after the first sector which, of course, wasn't much use for us.