32.50 * 32.55 I place a buy limit order @ 32.51 on ACRA and immediately get filled. Is this because there is a sell limit order on ARCA @ 32.51 that is hidden liquity?
You have to show at least 100 shares if you want specify max floor (aka iceberg or hidden liquidity). Which market center posted the quote above? Was it at NBBO? If not, your order got routed out. Or you just got lucky because you caught a seller that decided to step down a couple of milliseconds earlier - T&S would tell the story. If you got the fill from NYSE, you may have received a price improvement from the specialist since your order was marketable.
Also make sure you're order was a buy and not a sell, I took a second look at one of my orders that got filled instantly and I had sold short instead of buying. 99% chance this isn't the case but it can happen.
It could have been an ARCA discretionary order. That will act like an automatic "hitter" as soon as a bid is placed at its price. It also could have been a hidden NSDQ, BATS, EDGX, or EDGA. It depends on how ARCA handled your order. I frequently cross the market with ARCA and get filled on hidden liquidity. Short answer is there was already an order to sell at your bid price. (I never trade NYSE stocks, so I have no clue how the specialist fills...)