That's because it is completely reasonable, Andrew. Have you been living under a rock these past few months? Even if he had been, at this point, there's really no excuse for not being aware of the existence of giants. News might travel with comparative slowness in her universe, but the giant that fell in Jack's yard was there long enough for news of it to travel well outside the kingdom.

All that not being said, the idea of this being a dream isn't an entirely new one to her, and Greta's shoulders slump a little as that theory reasserts itself. She's not at all used to dreaming like this - to knowing she's dreaming - and it's easy to forget. Apparently. "Oh. Yes, that actually--I think that might be right. A, er, talking unicorn said much the same thing." She does sound faintly embarrassed about the unicorn; unlike giants in the sky, the thought of talking purple alicorns just sounds like plain nonsense.