Maybe your sample size is too small? I've lived close to the NL/D border for a while and the McD quality was indistinguishable on both sides of the border. The variation between restaurants in the same country and also between different days/times in the same restaurant was much greater than between countries.

Thatis, if you happen to go to a random McD in some country and the big mac was great that day and you go to a different restaurant in a different country on a different day and the big mac was bad, then that difference has likely least to do with them being in different countries. It's not like they actually use different recipes.