You are right, and volume (which can be compared to demand sort of) helps dictate the PPS, based on where people are willing to place the Buy and Sell orders. So where does the discrepency come in on ones that are printed as buys, vs ones that are printed as sales? Are the sell prints ones that are collected by the MM's, therefore increasing available shares, and technically lowering demand? And that would make buy prints those that move into retail hands? or are buys simply those that are 'moved' at the current ask and sell prints those that are moved at the bid, regardless of who buys?