He already knew Roxas as Ventus but seeing someone in a puppet who's face would resemble someone you are close to(enemy or friend), he's surprised that he's actually close to Ven. I personally think because maybe it's Ven who gave him his scar for life.
I'd sooner say he was surprised to see Xion as a copy of Ventus as opposed to Roxas who only had the face.
As to why he saw Xion as Ventus, regardless of what Sora had of Ven inside him, be it Ven's own Memories or Memories of him (i.e. Sora's Memories of Kairi that shaped Xion - note the difference).
Roxas's appearance was set at birth, so even if the Memories passed through him no their way to Xion, it wouldn't change how anyone saw him.
That is not the case with Xion who by the time Xigbar saw her as Ven had more of Sora's Memories than Sora and Roxas did together.