i think it’s because she doesn’t want to seem unknowledgeable of riftans accomplishments and skills as a knight and leader. the only time she saw him in action was when their carriage was attacked, but she was too terrified to pay attention. plus she’s been his husband for years, but her father hasn’t taught her anything about how to be the lady of the house nor basically anything helpful to her as a noble. she probably feels so incompetent when it’s not her fault, and i feel like she wanted to talk to the knights normally (since the other ones seem cold and intimidating to her) and feel some validation.
She's scared of passing of as an idiot in front of the others, and being judged for it. Everyone is treating her nicely which is a novelty in itself for her, but scares her at the same time. She feels like if she doesn't perform well, or drag down Riftan's name, they will throw her away so she tries to hide her shortcomings. Do remember she was abused by her father all her life and was treated like a mentally ill person because of her stuttering. She needs to build up confidence and work on herself before she can put that behind her.
I just don't understand why she kept on lying? Does it make her look cool?