The above may be plausible explanations, but exactly where and when does this information get used? When you have a real problem, you give the authorites the number of persons on board. You do not give a "plus" number, because they don't care. If time permits (and it generally does) you break it down into males, females, children, infants and crew. This is useful information for the RFF crews. Again "plus" is not used. This "plus" rubbish is a left over from the past and has no real purpose today.
PM