The point of having § 201(2)(a) and § 201(2)(b) is that you can have reason to know something and not know it. Imagine you have reasons to think Jones is not really your friend, but is in fact your enemy. But you do not want to believe it, and you ignore the evidence. You have reason to know, but you do not know. We can think of Embry v. McKittrick in either way.
The § 201(2)(a): McKittrick actually realized that Embry thought he was promising employment when McKittrick said, "Go ahead, you're all right. Get your men out, and don't let that worry you."
The § 201(2)(b): McKittrick did not realize that Embry thought he was promising employment when McKittrick said, "Go ahead, you're all right. Get your men out, and don't let that worry you." Imagine McKittrick was, for example, so distracted and preoccupied that he did not give any thought to what he was saying.
The point of § 201(2)(b) is to hold that McKittrick promised just as long as he had reason to know the relevant facts.