You don’t expect that someone will blow through a red light, but you expect that they could, and so you look both ways before you accelerate if you’re the first one waiting when the light turns green.

You wouldn’t expect someone to suddenly accelerate from a stop while they have a red light, so you don’t typically look both ways when driving through a light that has already been green for a while when there are cars on either side waiting.

I’m not saying Waymo cars do that; just an example of expected unexpected vs unexpected unexpected behavior.