Katniss has been here long enough that she assumes that the people she knew from her Matrix? She's never going to see them. It was easier during recovery to act as if they weren't real, just a part of the system, that what happened to them hadn't been her fault but just a result of the program that trapped them.

So when she sees Johanna for the first time, she doesn't know how to react. She'd tried to leave that all behind her, to start over, as hard as it had been. She'd been doing pretty good, too, all things considered.

So the words out of her mouth are disbelieving, a bit awkward, and even jarring. It's more a result of a lack of anything better to say in the spur of the moment.

"What are you doing here?"

It's not the greatest greeting in the world, but most aren't as far as Katniss goes.