Because i believe that Itachi has been giving sasuke jutsus sine they were kids. And way i say this is Itachi has been tapeing Sasuke on has forhead as a wayof giving him jutsus little by little.
thats wat i believe too.....very nice theory, you're a good observerbut I don't think it's that simple to give someone a jutsu... i think it was just itachi's way to show sasuke his affection