i just thought that, since all the inter-lang links are stored in the database, so either the links exist or not, I don't know why there will be one-way link.
They're not stored directly in the database, they're stored in the source code for the articles just like links within one wiki. The only way one wiki code detect the links to their article on other languages articles would be to read every single article on that wiki. Not practical.
The bots do quite a good job on the large wikis, I think.