This is a play on the literal meanings of Rikka and Touka's names, which mean "six flowers" and "ten flowers" respectively. (Were Rikka's parents trying to give her an inferiority complex...?)
A lot of the other Chuunibyou characters have numeric names as well: Tsuyuri means "May 7th", Isshiki means "one color", and Tsukumo Nanase's name contains both 99 and 7.