The trouble is this doesn't really prove anything for the exact reason you're identifying. What you have to compare is the level of enforcement compared to the level of compliance. If US companies are more compliant than average then there should be less enforcement than average, but then you still don't know whether there is more or less than there should be. If US companies are fined 75% as much even though they break the rules 50% as much, that's evidence of bias rather than a lack of it.
Moreover, major US companies are hard sticklers for following rules. They're full of lawyers because their home country is and they'll get their pants sued off otherwise.
That actually seems to be the problem. The EU passes some complicated stack of rules, the US companies have their lawyers go through them and figure out how to comply without disrupting their business, but disrupting their businesses was the motivation for the rules. Then the EU comes around and fines them again. Everybody who dislikes them cheers, but it smells rotten because the companies knew they were under scrutiny and had good lawyers, so it's difficult to find explicit violations and consequently huge fines get meted out over behavior which is not even clearly a violation of the rules.