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?

It's the same relationship between the food they serve in Mexican restaurants and the food that people actually make in Mexico.

Lots of recipes are changed to adapt to the vegetables available here.  There is a vegetable called kai lan that is sometimes called "Chinese broccoli," which is hard to get in the US and much more expensive than broccoli here, so it's broccoli that's used in the recipes instead of kai lan.  And so on.

Lots of the recipes are changed to suit American tastes, with extra meat and more fried stuff (Americans love meat and fried stuff, have you noticed?).  The spices are often changed too, like the amount of hot pepper reduced considerably (because Americans tend to have less tolerance for spicy foods than Chinese people do), and spices substituted for what's inexpensive and available here.  Saffron is really expensive so they might substitute paprika, and Szechwan pepper has been very hard to get in the US unless you buy it from a Chinese grocery store, so they might substitute cumin powder.  And so on.

And some dishes were created in the US and have no history and little connection to China--remember, the Chinese-American community has been here for well over 100 years now.  Things like chop suey and General Tso's Chicken aren't from China, they're from San Francisco.