Wholesalers have agreements with retail brokers to provide a certain level of &quot;execution quality&quot;, and that&#x27;s what they compete on (fiercely). If they pushed the price around on one order, they&#x27;d have to make it up on some other order to meet their obligations. It&#x27;s measured against NBBO at order receipt time, so chollida1&#x27;s example would count as <i>negative</i> price improvement (even if the best offer was only showing 100 shares and not 2000).