I think a lot of our confusion about deer eyesight has to do with our own perceptions. Eyesight is our primary sense, probably followed by touch, hearing and then smell. That's not the case with deer. Smell is definitely their primary sense presumably followed by eyesight, hearing and touch (at least that's how I'd perceive it). Their sense of smell is so much better than ours that we can't even begin to conceive what they pick up through their nose.
Because we can't comprehend that we may often mistake a warning from a stray waft of odor as being "seen". In reality that may not be the case.