Consider the fact that 85-90% traders lose. That is how they can cover those that win. And they are required to have enough cash on hand to cover.
Most retail orders are filled in-house first. Matching up buyers/sellers internally and going out of house if need be. And from what I understand, most retailers try to keep enough spare change on hand to cover as much interally as possible before ever going to the real market.
No different than a casino. The losers cover the winnings of the winners. The averages over time are definitely in the retailer's/casino's favor.
As far as re-quotes etc go... it's either re-quote or have a variable spread. Take your pick. Trades have to be matched up somewhere whether internally or on the market. Not an easy thing to do during NFP. The fact that a retailer says their spreads are fixed tells you right there that they take the other side of the trade because the REAL market there is no such thing as a fixed spread. In the REAL market, the spread is determined by the market itself.
Go read up here:
http://nondealingdesk.blogspot.com/