I have no evidence, but I've wondered if Vito's little diversion to the fruit stand didn't throw off the timing of the hit.
Well, the two hit men didn't emerge until Vito had already been at the stand for awhile. Had Vito gotten right into the car instead, the only shot the hitmen would have had would have been as the car drove by. But, of course, the whole scene was based on the novel and was designed for drama.
