It probably shouldn't have surprised her all that much. Luke was very smart. And intimately familiar with the situation.

And Obi-Wan probably treated him like an adult. Whereas she constantly felt like he saw her as a child.

Well, it wasn't like she was going to lie to him now. She nodded slightly. "Someone told your father some horrible lies about Dean. And he just..." She could feel warmth behind her eyes and she had to push it back. No more crying. She refused to let herself cry again. "...killed him for it. And I wasn't there and it's my fault..."