From the explanation what I understood is that the round fired is a rocket that is carrying the flechettes (like a fragmentation grenade). When you explode the round, the flechettes come out of it, and bypassing the shields damaging the ship.They are penetrated when the Flechette is detonated, so an explosion, not the flechette just passing slowly through the shields.
The rocket itself bounces of the shield (high energy, since it's a large kinetic projectile with self propulsion), but the flechettes bypass because they are smaller non-self propelled projectiles. Is that understanding wrong? I don't have one on me to read the in-game description.