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Does this have something to do with the latest update rumblings?
So nobody knows what the update was that took place on 16th May 2013?
It's an old data set, our site has over 500,000 pages and the URLs showing up in the index right now are pages we had removed from our site after first version of panda/penguin hit.
I believe this is the pattern:
Reset data to old
Reindex the web
Unleash the new algorithm on the new index
Could be 100% wrong, but believe this is what's happening.
It seems like if Google came back around and saw that all that web spam was indeed gone