Could it be number related?
Eleven is English. The same number in Japanese is ten-one.
Twelve is English. The same number in Japanese is ten-two.
Twenty-one is English. The same number in Japanese is two-ten-one.
Maybe this explains their superiority.
Is the systematic transparency of East Asian numerals one of the reasons why they perform so well in mathematics tests? Maybe. In one experiment, Japanese and American first-grade children were given a set of blocks to represent numbers. White blocks represented units, while purple blocks represented tens, so that one purple block was equivalent to ten white blocks.
The children were asked to read a number on a card and then to use the blocks to show that number. On first trial, American children tended to use only the white blocks (representing units), while Japanese children tended to use a combination of white and purple blocks, indicating that they had a better grasp of the decimal number system. Only on second trial did the American children begin to use the purple blocks as well.
In other experiments, Chinese and Korean first-graders have been shown to perform more or less like their Japanese peers. (Linguistics for Students of Asian and African Languages chapter 1 page 15)