Still waiting on response from broker, figured I'd throw this one out there and see what you guys think. 100 lot posted bid @ ARCA. 5 seconds later receive partial fill that provides liquidity, a few seconds later receive another fill that takes liquidity for the remaining shares. Order was not modified between fills. Any thoughts on how the first partial 'provide' liqudity while the second fill 'removes' liquidity even though the order was never modified?