I always thought DiZ was Ansem's heart. Before Ansem began studying darkness, he was a kind and wise man, like DiZ. Or at least, DiZ seems nice enough to me. Ansem himself is just Ansem's body, controlled by Heartless and stuff. It works, maybe DiZ won't tell anyone 'cause they'll be all 'Wtf?! Ansem?! DIEEEEE!'.
Or maybe he likes to be mysterious or whatever. He's definately connected to Ansem... he has Ansem's skin and eyes. In the KH2 trailer he's referred to as Ansem, and Mickey says in CoM he's met DiZ before, though he doesn't remember where.
Oh yeah, Zexion mistook Riku for DiZ when he first entered the castle, because their scents were the same. Maybe that's because Riku's scent would be Ansem's, because Ansem is in Riku's heart... meaning DiZ and Ansem have the same scent. o_o