I don't know if it's the sugar or not but, to me, there's a significant difference between the taste of Mexican Coke and American Coke.
A former neighbor of mine worked in the Coca-Cola lab in Los Angeles (during the time of the "New Coke" debacle) and he told me that they used different formulas even for different sized bottles. He said that he only drank the 8 Fl Oz bottles because they contained the best mix, so it's possible that the bottlers down in Mexico use a different formula than US bottlers. Mexican Cokes tastes less "sharp" in my opinion; perhaps they use less phosphoric acid?