You can't be that dense. Even if that really is what's underneath the model, and I won't doubt that it is, that's because they just used Siegfried as a base when they were creating Nightmare's model, because the sword is drawing from the memories of when it was possessing Siegfried, so it was a developer shortcut.
The difference, though, is that when NIghtmare in SoulCalibur IV, in-game, has his high equipment broken, the mask doesn't get blown away so that you can see this face, it just gets battle damage. On top of that, when you make a customized Nightmare, it's not Siegfried's face you see when you're doing it like it is Raphael's face in SoulCalibur V, because it actually is possessing Raphael's body in SoulCalibur V. And Nightmare's high equipment broken in SoulCalibur VI reveals Siegfried's face intentionally, because this NIghtmare is possessing Siegfried's body.