This is a Statistics Canada process. The question is interesting and that is why we also asked them that. According to what the Statistics Canada experts explained to us, in their statistical survey, they consider that mother tongue is a significant indicator of the minority language population, in this case.
They carried out their statistical survey to attempt to verify correlations and so on. That is why the figures exist. Basically, Statistics Canada experts compile these data because they do calculations pursuant to their own statistical verifications, and these are significant numbers that mean something.
All of that said, one has to see their meaning when attempting to explain them. That is why I was taking the time to specify that there are different terms and different definitions. Depending on the debate one is having, some figures are more relevant than others.