Imagine if you will a market maker working on any Major desk. In the the bank of buttons and screens he has for quick access to anything, he has the abilty to appear as 1 or many bids or asks but the act of trading through another market maker often has a Spinning effect of having the Beard begin to play on the Major's tickets ;)
The advent of the Archipeligo trading island allows a Major to place Bids or Asks at odd sizes but with the patina of Internet privacy. It has been strongly suggested that often an Error Account was actually the ARCA client!
That being said, when you are a Long and you see ARCA in the deal urgent and often "especially at month end" its usually a good sign. ARCA is rarely an enormous seller of anything ;)