MrSpeed - 4:36 pm on Oct 30, 2003 (gmt 0)

I was going to ask this exact same question myself.

I have seen so many different theories that the only conclusion I can come to is that I don't think anyone knows what the pattern is.

Here is my 2 cents:
The displayed PR and backlinks are updated at different times than the PR and Backlinks used for the SERP calcs.

New pages/sites are very quickly indexed and rank well if linked to from a page with decent PR. The new pages then disappear from the serps until a permanent listing appears.

Some believe there is a three week cycle, but there never seems to be a consensus when it happens. Maybe the index is split into different groups and each group updates at a different three week interval?