Perhaps because he hadn't formed a theory yet to explain Lex's choice, Adrian is surprised by the explanation. It makes sense, of course, and he's both touched and proud that Lex would think to mark the occasion in some tangible way. It's simply unexpected -- gifts, in Adrian's experience, nearly always come with expectations attached.

"You're right. It does deserve to be recognized, and you have impeccable taste in monuments, Lex. Let me return the favor -- give Aimery the night off and be my guest for dinner. I know an excellent place."