Personally, I interpreted her surprised expression in this scene to be a form of "soliloquy" for the audience's benefit. ie: Something the audience sees, but other characters in the movie don't.
The real surprise was Lousie-in-present experiencing her first clear, lucid "memory" of the future, and they showed this by having Louse-in-the-memory act shocked at what she was "remembering" even as she spoke to the ambassador on the phone. In truth, Lousie-in-the-future would not have been surprised, because she would have had all the time between "present" and "future" to prepare for that event. The facial acting (or "mugging") by Mrs Adams was for the audience's benefit alone.
Of course, this is just my opinion. I would be surprised if the screenplay or any other official source explicitly addressed this point, although if such an official source can be found, I'd be interested to see it, too.