Volume is reported differently on the Nasdaq vs the NYSE. This is because of the Market Maker system on the NAZ. When there are trades between two market makers, both report the volume. When one of us individuals places an order with a market maker only one entity reports the volume. Therefore, a block trade between two market makers of 600,000 shares really involves only 300,000 shares.
It does not appear anyone on this message board knows this information. (I was a stockbroker from 1982 to 1986).