the process in processed cheese allows them to go as light as they want with the real cheese, I'm sure some government regulation has a minimum but the core point is that not all American cheese is the same

For the formal legal definition, American cheese can only be mixed with water and cream, such that the fat from the cream is less than 5% of the total product.

They're allowed to add:

• Water but the total moisture content of the cheese can't be more than 40%, and the fat content must be at least 47% of the solids (that is, the non-water part) in the total.
• Cream or milkfat but not so much to where fat from this cream/milkfat exceeds 5% of the total product.
• Acidifying agents but have a minimum pH
• Salt but it still needs to taste good
• Spices and flavorings that don't taste like cheese, but still need to taste good
• Smoke but it still needs to taste good
• "Harmless artificial coloring" which by its nature isn't going to constitute a significant percentage of the total weight
• Mold inhibitors up to 0.3% of the final product
• If sliced, lecithin may be added as an anti-sticking agent up to 0.03% of the final product.

Basically I can't think of a way to make stuff that can legally be called "American Cheese" without using at least 90% cheese as an ingredient. If you cut it with too much water you'll run afoul of the milkfat and moisture minimums. And everything else you're allowed to add is never going to constitute more than 1% of the end product.