"Romana." The Doctor all but whispered her name, his eyes going wide from surprise. She was one of the ones he was sure had died, and she shouldn't be there. She couldn't be there. So, The Doctor was confused, but at least it wasn't the first time for that. Or even the millionth time.
"How did you... Why are... Wot?" He ended up falling back on his standby question. When he got confused he had the tendency to ask "what" over and over. And over.
Part of him still felt like she couldn't be who she said she was, she just couldn't. But the slightly more rational part of him knew that no one else would know of that conversation about their names. It was so long ago, and most find it much too trivial to remember even if they did hear about it, which they probably didn't.