The whole point of that fight is that sif gets weaker the more you hit him. He does the opposite of what a boss is supposed to do. He even starts limping and can barely move towards the end if you dont kill him too fast.
Story wise Sif is an innocent and is only trying to protect you from abyss which is what killed artorias. So it's supposed to have an element of tragedy to it.
Oh I see. Too bad I play the Souls games in Jason Vorhees mode. Didn't really noticed that detail, maybe I just assumed any weird stagger like behavior was because of my Iaito +10 bleed effect.