Yeah, that's a puzzler too, but 121 shares may have been a partial fill by a MM for a previous trade.
The most obvious conclusion I can come to with this trading is that those sitting on the Bid looking for cheaper shares are the ones who are holding the pps back. The MM's look at the supply and demand to determine their Bid and Ask prices and when you don't have anyone buying at the Ask, they lower their prices to sell to those on the Bid.
But then, that's only my opinion.
Market Data 
Markets 
AI
