Well that depends. Is it just random Youtube videos? Then I would say, that's why.
A lot of people do not have the general knowledge of how sales work. Even I didn't and still in some ways could further increase my knowledge. I think in the PS4 100M thread I thought it would only get to 105-110M. 115M max. Now I know that's completely unrealistic. And when I joined this site, I had this idea that kind of carried over from childhood that Zelda was Nintendo's good ol' second or third best, after Pokemon and Mario. It wasn't like some kind of thought-out belief, it's not like I was 10 and saying "I bet Zelda sells almost as much as Mario!". But the way the series, and in particular Link, were marketed made me believe that the franchise was one of the biggest of Nintendo. Obviously, with BOTW now things are different, but that definitely wasn't the case when I first found this site. Edit: (It might be if you include ALL software sales, but obviously we're talking about the impact of the series from game to game).
Kind of an odd example but you get my point. People don't have general know how and I still find there's a lot I need to learn about sales and a lot I still do learn everyday from some of the users on this site.
Edit: I voted 130-135M in the poll.
