A recent report says that Yahoo! has updated the Yahoo! Web Search index. It was on March 28th when they last updated their index. However, it was conveyed by Yahoo! Product Manager that these updates have been "occurring more frequently; this is the result of improvements to the indexing system." Due to this, you may also experience some changes in the ranking of the pages which were included in the index.
To know more about it Refer Yahoo! Search Blog.