He asked for a small/private party, but she said everyone would be there... only to actually have a one-on-one (maybe Touma may have turned up, but I only remember seeing 2 plates). He'd be surprised that it was a private party after all, but that's only a happy surprise because she tricked him and turned down his original request in the first place. That makes the expectation for a large party artificial, and a relative disappointment.