i think roxas was one of those special nobodies.... coz remember at the end of 358/2 days roxas cried coz xion was dying?? well nobodies are not meant to have hearts.. so my theory is that half of vens heart was in roxas, which i believe half of roxas was ven and the other a nobody, and the other is in sora so that he can still wield the keyblade.. so i think everything will be explained in the upcoming game.. i think its gonna be called Reconnect Kingdom Hearts (thats what the secret ending said in the scene with sora kairi and riku where sora has decided what to do after reading that letter)