This is the specialist being unable to match the institutional blocks of stock. 70k share on a high volume stock that trades 100m shares isn't much. On a million shares traded stock, on the other hand, is significant and will gap the stock up/down the following day. That outstanding balance will show up on the specialists book the following day and will impact the opening price. Sell imbalance and the stock gaps down (depending on how many bids are in the book the following day) Another example; mrkt futures open up +200 the next day and that imbalance won't matter.