This doesn't really surprise, especially for anyone who gives a thought about it.
Take for example a free software product. On BitTorrent, you'll likely find just the most recent versions and in a few different languages if the product has been translated. There also tends to be just a few main trackers for these Torrents.
Now, consider an illegal distribution such as popular TV series. There will be Torrents for every individual episode, Torrents for each or multiple series as well as individual episodes and full/multiple series for each language. Of course there will be different recordings shared out across a wide range of public and private trackers, including different recordings in standard definition, DVD-rips, 720p, Full HD, Blu-ray rips, encodings specific for the iPhone or PSP, etc.
So for each legal file on BitTorrent, there are likely just a few different versions on it, where as for each illegally distributed file, there are potentially 1,000s of version of this same thing.