The Auror starts dodging as soon as he recognizes the Killing Curse. This probably gives Quirrell a reasonable window of time to judge whether or not his opponent will dodge in time.

How large this window is, that's another question. Voldemort can say the incantation in "less than half a second" but the description of that doesn't match the description of the Azkaban scene, so it seems that Quirrell says the curse more slowly. This suggests that either he is telling the truth when he says that he wanted Bahry to dodge, or else Voldemort's skill at tongue-twisters is rare even among capable battle wizards, and he doesn't want Harry to make the connection.