I often wondered about that.

We hosted an exchange student for a few weeks, and he was from Nanjing. Before he left the country, we took him to a Chinese restaurant and warned him that it was likely going to be more like American-Chinese.

He went through the menu and pointed out the dishes which were authentic and those which were not. I was surprised at how many were actually authentic -- it was about half of the menu. Maybe we were at a more authentic Chinese restaurant, as the menu was in both English and Chinese.

He was a great kid, and I really enjoyed the experience. He loved peanut butter and jelly, had to spit out ranch dressing, and did not care at all for pumpkin pie.

There's also the question of authentic/traditional to which part of china, in particular in cases where dishes with the same name aren't made the same. But beyond that, just because there's a dish on the menu one recognizes from their homeland doesn't mean it's prepared the same.