Damien's blank expression wasn't what Katherine expected to see, making her wonder as to what was going on in his head.
Years ago, before she lost her memories, she told him she was going to tell him what 218 meant for her but she never had the chance to, even when she met him again after that auction. Things happened and they never got to talk about it. And now that she thought about the past, she mused as to why she even shared this part of her to him in the first place. And to some stranger on top of that!
Out of all the lies that she said back then… Out of all the different personas that she had put on in front of people outside of Shadow, her birthday was the one thing that she held so dearly.