Sorry, 757SB, no offence meant.
It was aimed at those, unlike ourselves, who want the representation and all that comes with it but are not members.... yet!
And BALPA don't know - for sure. They don't hold a database of all our pilots and who is a member or not. They rely on members like us to help them out on that score and give them the info. And things change, so we need to tell them who leaves and joins. In a workforce our size, a move of, say, 10 people can make a big difference if they are members or not. Think of the 73' and how much the trim changes if you move very few people front to back or vice versa!