I just bought 500 shares (taking liquity) of a NYSE stock, on ARCA. Where there were 2000 shares available on NYSE.
This is probably why: ARCA offers lower take fees and greater rebates to Firms that trade more than 100 million shares a day through ARCA, http://www.nyse.com/productservices/nysearcaequities/1157018931977.html So IB wants to execute as much volume through ARCA so it can hit these volume tiers so it gets lower take charges and greater rebates. Because IB doesn't pass thru their actually ECN Fees and Rebates to it's customers, they use it a another source of income. Also if you look at the link above you will see that if ARCA routes and removes from the NYSE book it is only a .001 fee. But IB charges you .003 ECN fee, so they pocket a .002 profit off what they charge you for that ECN and what they actually pay. http://individuals.interactivebrokers.com/en/accounts/fees/ARCAstkfee.php?ib_entity=llc So IB probably more than doubled their profit margin on that trade by doing this. However, most retail BD's do this, even Prop BD's also do this.