Several things are different.
Many of the Chinese dishes use ingredients that are not readily available in the US, although in some areas with a large Asian population many of the ingredients are more likely to be available these days. The Americanized version uses substitutes or simply skip the traditional ingredients. And then there are difference in HOW it is cooked.
For example, real Kung Pao Chicken uses Sichuan Pepper which is not a pepper or related to chili pepper. Plus, from 1968 to 2005 the US FDA banned the importation of Sichuan Pepper. The Americanized Kung Pao Chicken uses chili pepper and black pepper. The Chinese version is very spicy whereas the American version has almost no spicy flavor.
The peanut used in the Chinese version is not just raw or roasted peanut tossed in and cooked, but is first dried and then fried in peanut oil before added to the dish. In some place in the US, because of concerns about peanut allergy, peanuts are omitted.
Then there is the cooking wine which in the Chinese version is a specific kind call Shaoxing wine, but in the American version sherry is used and in many case not at all.
Also Americanized versions tend to be too sweet and over used of sugar. The Chinese version tend to use herbs and spices more.
In terms of seafood dishes, Americanized versions are usually fried to eliminated the "fishy" taste. The Chinese LIKES the fishy flavor of seafoods and often complain that western seafood dishes are bland.
Western meals tend to have more meats, whereas in traditional Chinese meals, the vegetable dishes number more than the meat dishes.