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?

As above posters said it has been adapted to better suit it us here in the US. Its just a personal improvement designed for our tastebuds. Personally, I can't stand the fast Chinese food. I cook szechuan recipes here at home (real szechuan)and I think they are absolutely wonderful.