petehall - 10:31 am on Apr 21, 2012 (gmt 0)
@tarzan007 Matt Cutts announced that they 'lost' a large number of websites during the update. So the data (we think) has been rolled back to before this loss.
And a new index is being built with the data included. When searching, the results are switching between these two indexes.
The way to tell is look at the number of results returned on a search. You should be able see two very different figures if you try the same search on different browser sessions.