Maybe they are compatible with the new tubular-clinchers. Those are a clincher-mounted tire that is supposed to offer the advantages of tubulars. See http://www.tufonorthamerica.com/whattype.php and http://www.tufonorthamerica.com/tiretypes.php. I've heard they are very hard to mount and generally not worth it. There are also "open tubulars" that are basically tubeless clinchers that are supposed to ride like tubulars. A rim designed for them would need to be leakproof for air, or designed to hold a leak-proof liner. I'm guessing you've got a rim designed for one of these new styles.