The amusement is, at least, evidently neither unfriendly nor even quite mocking - Martel doesn't disagree with him, after all. It'd have been politer if he kept it to himself, given the way he demurs from entering conversation on the subject (a conversation that would be more interesting to have, he thinks, after Mycroft has been here longer), but if he were routinely courteous people would become concerned for his health.

“The latter,” he says, instead, holding his two lists up for comparison with a slight frown; it'd also be politer if he paid more attention to the person he's actually in a conversation with, but given that what's occupying him instead is intended to be useful to said other person, progress of the world will likely not grind to a halt over it. He's not uncivil, just inclined to treat manners like something useful when convenient and otherwise irrelevant to him.

(A clear aristocratic origin suggests altogether too many possible reasons for that for any one particular to present itself. At the library, he's sometimes referred to as his lordship; at Hellsing, he is invariably Sir Martel because he had a knighthood, once, and Sir Integra prefers it to 'my lord'. The difference doesn't appear to outwardly concern him, and he rarely objects to simply being addressed by name.)

“Statistical history is slightly out of the way,” he says, stepping out from behind the desk with clear intent to lead the way. “I've a list for you here of the resources you're best to start with, but I'll show you where you want to start looking for them.”