Because he's a lawyer cross-examining a witness.

Video: http://www.c-spanvideo.org/clip/4456141

From TFA: Mueller initially sought to downplay concerns about NSA surveillance by claiming that, to listen to a phone call, the government would need to seek "a special, a particularized order from the FISA court directed at that particular phone of that particular individual." Is information about that procedure "classified in any way?" Nadler asked. "I don't think so," Mueller replied. "Then I can say the following," Nadler said. "We heard precisely the opposite at the briefing the other day. We heard precisely that you could get the specific information from that telephone simply based on an analyst deciding that...In other words, what you just said is incorrect. So there's a conflict."