You make a very good point that comparing american total aggregate scores to the best of the individual european states is misleading. We could compare total american average results against total european and show that the US does better, as you have, or even better is to present histograms showing american state by state results and comparing to european state by state results. This makes sense and is a fair way to compare since in both europe and the US, educational policy is mostly determined at the individual state level rather than at the union of states level.