What is your Octane rating on Regular and Premium there...that could be the difference.
In U.S., the 'Octane' rating posted on the pump is actually the AKI (Anti Knock Index), which is the average of Motor Octane Number and Research Octane Number...so AKI = (RON + MON)/2
GM specifies 87 AKI minimum for the 5.3 and 91 AKI minimum for the 6.2 in USA.
Here is a good description of the various Octane Rating Methods used/posted in Europe vs. USA. Note, I don't agree w/ the sentiments in the article that lower Octane Fuel is Lower Quality...it is simply less resistant to detonation under compression than higher octane fuel...whether this matters in a given application is dependent on engine design...but the article does get to how different countries use the different test methods.