How could you say such a thing?

My parents currently have another houseguest, the daughter of a friend from St. Petersburg. Poor thing is already caught up in my family's dysfunction (for example, she witnessed the "you've gained weight" convo described in the previous post). Then there was this:

A.L Where am I sleeping?
Mom: In our room. Dad's sleeping on the couch in his study.
A.: Why don't I sleep there? That way he can sleep in his own bed.
Mom: Well, he goes to bed later...
A.: I'm just saying, I don't mind sleeping on the couch.
Mom: I don't understand what you want.
A.: I'm just trying to minimize disrusption. I want everyone to sleep in their own bed.

Later

Mom: How could you say such a thing?
A.: What are you talking about?
Mom: That you want to sleep in your own bed.
A.: I didn't say that!
Mom: Now Anya feels awkard. She offered to sleep on the couch.
A.: I said I wouldn't mind sleeping on the couch.
Mom: Why are you getting defensive? Just admit it!
A.: Admit what?
Mom: That you said you wanted to sleep in your own bed.
A.: Because I didn't! I offered to sleep on the couch. In that context, it should have been clear that my comment about wanting people to sleep in their "own" beds did not apply to me, since that bed is not mine.
Mom: [Shaking her head].

I didn't add that if anya felt awkward, it's because Mom was being a bitch. The irony of this is that I can't be sure because I can't keep my parents' friends straight, but I think she's the daughter of the man who spent many, many hours on a train to visit my parents at their dacha only to leave the next morning because my mom had chewed him out for stepping on her flowerbed.

Later still, during the making of the matza ball soup:
Mom: The celery is chopped to coursely!
A.: Okay, I'll fix it.
Mom: You don't remember where the grain is, do you?
A.: No.
Mom: I didn't think so!

How difficult would it have been to say, "could you please chop the celery more finely, and by the way, the grain is in that container"?