Language statistics can be done in various ways, for different purposes, but most people just don't get the differences.

Mother tongue indeed is different from language of use. You mother tongue is Cantonese, but the language you use is English. Most immigrants and their children have different mother tongues, but if the children go to school in Canada, their language of use will eventually be English or French. 

there is also the "language used at home", and the language used outside the home...