I have found there are some supplemental result by siting my directory. The reason may be that I took a common category dump and accepted some submissions with the same title and description like others. There days I am working on modifying my directory, making all the category unique and searching friendly, and I should thank to all the people share their unique features. Now I find the supplement results are decreasing, so if you found your directory got same problem, you could make something special or unique to get it back. Some reading about Supplemental Result.