Why is this celebrating the 15th anniversary? The series wasn't 15 years old in 2003/2004?

For those not in the know, the screen above appears before the intro, and the "15th" logo appears on the covers for the console versions. The problem is that neither the original Street Fighter or Street Fighter II was 15. The Japanese version was released in 2003, take 15 years off that date and you get 1988 -the first Street Fighter game was released in 1987. Take 15 years off 2004, which was when Hyper Street Fighter II was released in America and Europe, and you get 1989 -two years after the first Street Fighter, and two years before Street Fighter II.

One possible theory is that Capcom wanted to get in on the fact that a rival series -SNK's The King of Fighters -was having a 10th anniversary celebration in 2004 and they wanted to get something Street Fighter related out in retaliation. Other than that, I've no idea, unless Capcom just made a mistake with their history.