US Chinese food vs Real Chinese food

I've hear people that were born in China and come to the US to try our Chinese restaurants say that they're very different compared to the food that's actually served in China. 

Can someone explain the differences in our perception of Chinese food and what's actually over there?

If you go to cities with a large Chinese community, you'll find real Chinese food. Another indicator of real Chinese food would be to look at the clientele. If you see Chinese people in the restaurant eating, then the place serves authentic Chinese. If you see non-Chinese, then it's an "American Chinese" food place.

In my city, though, "American Chinese" restaurants are dying out. People here want authentic (yet fast) Chinese food. So, a lot of noodle/BBQ places (char siu, siu ngap, wonton mein, hor fun, etc.) are popping up.