The top search result for prologue on Google is Matt‘s Prologue theme announcement. That same announcement post is number 5 on Yahoo! and on Live Search / MSN Search it doesn’t show up on the first page of results at all.
Feel free to speculate on those data points.
No related posts.
I speculate that the three search engines use different algorithms to determine relevance, and this can and often does result in different results being returned. The end.
@Joe -
Hehehe, I was going to suggest something about Live Search not indexing blogs very well (or at all?), but your theory works too