What exactly was Kirk responding to when he said "That's weird" after Gaila said "I love you"? I do not think he was responding to Gaila's comment.
I wonder, too; and I'm not seeing an answer here that rings a bell for me. I tend to agree that he's not listening to her, and it's something else.
I checked the novelization, and it's not in there either; it's a different conversation.