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?

All things adopted by america's mainstream are changed a lot from their original format. We cant handle things that are werd or different even though we blilndly follow the ideals of proggressive socialism.