If you buy a 25c of any other brand it will work. Specialized puts a 25c tread on a 28c casing. Look inside the tire there is a stamp that says 700 x 28c. They put a 28c tread on a 32c casing. They actually have to dual rate some racing tires as 25/23c so people actually know what they are getting. I had the same problem with Armadillos on a 1989 Diamondback frame. I wish they would label the tires properly.
Link to Specialized Roubaix tires with dual rating.