Apparently, if you publish a copy of something already listed, they will price match it to the lowest price of the same item. So if there is one listed as free, yours may be adjusted down.
So for example, this guy sold 4042 copies of War and Peace. But he only made $2.87. That is because Amazon price matched it for one that was priced at 0.00. So he obviously sold just a couple before they went in and made the adjustment.
I think there was a WSO that also talked about Public Domain works and how to use them to publish. I am also sure that not all PD works may have this happen.