Well, think about it this way: Clow was a very, very old man by the time of his death. A man who has even just one child several hundred years ago will, provided that that child has a child and so on, have many descendants. (And if the original man can travel through dimensions, it's not surprising that some of his descendants could have traveled through dimensions, too, and started up families in other worlds.)

Likewise, Li (more specifically 李) is a surname possessed by over 100 million people in China (if Wikipedia is correct). If it's so common in this world, why wouldn't it be common in a world so similar to ours?

(I'm sorry about posting near midnight, but I'm just wrapping up the IBARW links and and I noticed this on my f-list. Thank you for the translation!)