De difference is, I think, that Latvian girls are rarity. They are like pearls on the deep sea's bottom-line. You can find lots of various species in this sea, including very attractive species. But you won't find a better girl than a Latvian one.
It doesn't need any proof. It's a phenomena. Just come to Latvia and check it yourself. Any girl growing in such conditions like Latvia, is very different.
I never have been to Mexico, but I'm sure the girls there have different skin color, eye, hair, asses

However the public difference is only one aspect.