Oh, lying by omission. Of course she knows what that is, it was basically all Ciel did during her undercover time in Japan when she pretended to be a third year student.

...That is. She's all ears and no judgment, here! He can react however he wants, she's still trying to piece together the sort of 'argument' they may have had. Though from how Howl is making it sound, why is she getting the impression that he's mainly upset about not being told something specific he feels entitled to know?

"That's... Still a bit vague, I'm sorry." Her brow knits in deeper concentration. Now how to rephrase this tactfully... "Is it like now that you know more, you feel like what you've exchanged about yourselves between each other before wasn't of equal worth?" He felt cheated or shortchanged by this lie of omission, in other words? Is that the sort of spat this is?