I would gather we have much more certainty about Deep Blue's algorithms considering that we built them.

And you'd gather wrong. Our confidence that the woman says "hello" (and a fortiori our confidence that she does not take a gun and blow the man's head off) exceeds our confidence that Deep Blue will make a particular chess move in response to most common plays by several couple orders of magnitude.

You're getting into hypothetical territory assuming that we can obtain near perfect knowledge of the human brain & that the neural state is all we need to predict future human behavior.

We started off well into hypothetical territory, back when Stuart brought Clippy into his thought experiment. Within that territory, I'm trying to steer us away from the shoals of irrelevance by countering your hypothetical ('but what if [insert unlikely scenario here]? see, humans can't be predicted sometimes! therefore they are Unpredictable!') with another hypothetical. But all of this still leaves us within sight of the shoals.

You're missing the point, which is not that humans are perfectly predictable by other humans to arbitrarily high precision and in arbitrarily contrived scenarios, but that our evolved intuitions are vastly less reliable when predicting AI conduct from an armchair than when predicting human conduct from an armchair. That, and our explicit scientific knowledge of cognitive algorithms is too limited to get us very far with any complex agent. The best we could do is build a second Deep Blue to simulate the behavior of the first Deep Blue.