That's how my pair have lined out so far. Males heads when viewed from above are a lot more "rounded" and the females appear to to come to a point. My male is also a lot more bulky than the female. Mine are a bit smaller than yours, only 5 inches +/- SL, but I don't expect to see them change shape too much before reaching sexual maturity. From what I've read here and other sites, they can be differentiated from a very small size.
Impossible only means that somebody hasn't done it correctly yet.