Cal thinks Sherlock is doing a fairly decent impression of someone who just wants to get through something he considers unpleasant without upsetting his brother. That's probably what he'd mistake the act for if he didn't know better.

But he does know better, and the subtle indication of support is reassuring. He is, for a change, not in this alone. He relaxes a little, and the smile he gives Tony is convincing as he says,

"I don't know, Uno does a deep dish that's pretty good, you ever been?"