A thread at WebmasterWorld named Slowest Search Engine in History?: like watching paint dry discusses what many have been saying for a while, Yahoo! is ranking Web pages based on old scoring data. Let me be clear, I do not work at Yahoo!, so I do not have hard facts, but if you read forums often, as I do, you will notice people complaining about Yahoo! being slow to update certain scoring elements. Some examples of issues reported include; ranking sites based on linkage data from 5 months ago has been reported and not updating 301 redirected pages for years on end.
There are fluctuations in the SERPs at Yahoo! but when looking at individual pages and sites, often you see a huge lag in the criteria they use to rank the pages. mfishy puts it well with this line;
This does not mean that Y! are not crawling and even indexing new stuff, it means they are largely scoring pages based on either old data or their editorial staff placements - neither gives me great confidence that they will even be able to hang on to their 20% search share...
The ranking algorithms are tweaked, but the data used in those ranking algorithms seems to be old.
Forum discussion at WebmasterWorld.