People should bear in mind that most ads are made by outside agencies, not the home company. It's very likely that these ads were not made by Nintendo themselves. That being the case, it's entirely possible that the agency doesn't know how many characters will be in the finished game, especially since Nintendo are keeping such a tight lid on it. All they would know in that case would be that there are 40 characters now (including the three Mii types as different characters) and that there are going to be more. So "More than 40" is the only statement they can really make.
But really, the whole point of this thread is kind of moot. Between the leak and the official reveals, we already have over 50 characters. The three Mii types will all officially count as separate characters, in much the same way that Squirtle, Ivysaur, Charizard, Zelda, Sheik, Samus and ZSS all counted as their own characters in Brawl despite only taking up 3 CSS spots between them. So the roster is already more or less confirmed to have more than 50 characters. The advert saying "more than 40" really means nothing.