Quote from hughb:
Back in the 90's there was some squabbling between the NYSE and the Naz over volume reporting. The Naz was reporting more volume than the Big Board so the NYSE complained that the Naz was inflating it's volume reporting by reporting a transaction of 100 shares as being 200 shares in that a buyer bought 100 and a seller sold that 100. I don't really remember for sure, but I think the NYSE started changing how it reported volume as well. I did a quick google, but I couldn't find anything definitive in the first few hits.