Quote from ammo:
is that because of the fake bids/offers entered /pulled by the hft's
No--anything related to volume has nothing to do with orders; volume is a done deal, no hiding possible (at least volume reported by the exchange, this ignores any dark pools and all that stuff).
One reason for the variation can be described in this scenario: a transaction occurs at 51.75, and then a market sell order takes the last traded price to 51.50. Then at high frequency the bid and offer "rotate" around 51.50, so that the market goes 51.25/51.50, to 51.50/51.75 in millisecond timing, and sellers only sell when the inside bid is 51.50, and buyers only buy when the inside offer is 51.50. Below is a picture of this happening a moment ago, and it happens all the time, all day. It happens so quickly that you never even see the bid/offer switch on the DOM, but that's what's happening in super sub-second time.
So, the first time it ticks DOWN to 51.50 this time it's counted as sell volume (negative delta). When the above scenario happens, all subsequent transactions at 51.50 are counted as sell volume, since the market has not ticked UP, EVEN IF there are buy orders executed. If the market had ticked UP to 51.50 from 51.25, then all subsequent transactions are counted as buy volume (positive delta).
In a bid/ask delta calculation, a buy market is counted as positive delta, and a sell market is counted as negative delta; the market price and which way it ticked is irrelevant.
There may be other reasons for the differences but this is one.
http://screencast.com/t/X6tsqgnY2U