robertskelton - 10:56 pm on Aug 27, 2003 (gmt 0)

Many resulting pages have since changed or are now dead, and they tend to come from large sites, so I figure the supplemental results are from an old index, an index that looked at every page in large sites, rather than just most of them.

My guess: whenever Google has some spare spider power, and time up their sleeves, they go find some extra pages at big important sites (like blogging sites). The data is supplementary because they never know when these pages might be indexed again, if ever. They know the data is old, so they don't include it in the main results, unless it looks like it could help.