[quote=Lalliman]
I've considered that too. Being somewhat of an anti-climax boss makes sense for him, considering he doesn't have a lord soul anymore.Then again, Ornstein, who is both mechanics- and lore-wise weaker, is unparryable. It's just a matter of gameplay and story segregation, i suppose.
[/quote]

Technically, Lore-wise, both Ornstein and Smough, along with their Souls and item Drops, were illusions created by Gwyndolin. You fight the Old Dragonslayer in Dark Souls 2, who has the exact same armor as Ornstein, albeit a darker shade, and drops the Old Leo Ring.

If the one in Anor Londo is an illusion, then it would have made sense that he would be unparryable, since the Chosen Undead must be tested to see if he/she deserved the Lordvessel.