The Yahoo! Search Team has answered the call in a recent blog post, saying that a product fix was released:
While the counts have been incorrect in some cases, the actual returned results have been correct. However, we did roll out a product fix yesterday and will be rolling out a couple more over the next few days to resolve this difference in counts some of you have observed.
Thank you, Yahoo, for reading the Search Engine Roundtable. ;)
Forum discussion continues at WebmasterWorld.