You’re getting the labels mixed up.

As a labeling requirement under U.S. law, anything labeled “American Cheese” must be pasteurized process cheese made from some combination of cheddar, colby, washed curd cheese, or granular cheese, which the law also defines pretty strictly. It must be made from these cheeses, heated and emulsified with an emulsifying salt (usually sodium citrate).

American cheese is allowed to have some optional ingredients and still be labeled American Cheese:

• Food safe acid (as long as pH stays above 5.3)
• Cream or milkfat, such that this added fat can account for up to 5% of the weight of the finished product.
• Water (but the total moisture content of the resulting product must still be within the other limits in the regulation)
• Salt
• Artificial coloring
• Spices or flavoring that do not simulate the flavors of cheeses
• Mold inhibitors from sorbate up to 0.2%, or from proprionate up to 0.3%
• Lechitin, if sold in slices

You can add milk, cream, buttermilk, whey, or certain other dairy products up to 49% of the finished product, but then you’d have to call it “Pasteurized American Process Cheese Food” instead of just American Cheese.

American cheese is made from almost entirely cheese ingredients. The individual slices being sold at the store, though, vary by brand on whether they’re even trying to be American Cheese (or whether they’re some kind of cheese product, or even something less).