Did a team count and you are also right, funny how they call it the Big Ten.
Tradition. It's the oldest conference in college athletics. It adopted the name in 1917, kept the name between 1946 and 1950 when there were only 9 teams, and still kept the name when Penn State joined in 1990.
Take a look at the conference website. Look carefully at the logo in the top right hand corner and you'll see the number 11, acknowledging the number of teams. (Hint: look on either side of the 'T' in Ten).