http://goranlab.com/pdf/Ventura%20Obesity%202010-sugary%20be...
(Linked from here:
http://healthland.time.com/2010/10/28/study-hey-hipsters-mex... )
Explains that the Mexican coke tested had fructose and glucose in it. It could have been bottled with HFCS or perhaps the cane sugar split in the bottle.
That doesn't in any way account for all the possible differences with Mexican coke though.