In the pit, trades are not time prioritized. Thus a floor trader (local) joins the bid or offer and "leans" on the customer orders that are resting at those prices. Through relationships with filling brokers, when market orders are entered, the local tries to buy the bid and sell the offer. The most egregious impact this practice has on both customers and other locals is when the market "turns". For example, lets say for the sake of argument that SP trades in .25 like ES does. Of course it actually trades in .10 but lets suspend reality. Lets say the pit is 25 bid at 50, 200 by 10. You as a customer need to buy 50 contracts. Your broker lifts the 10 lot offered and now bids .50 for your 40 remaining. Other buyers move up and join the .50 bid, leaning on your order. Now an order to sell 20 lots enters the pit. You are not at all guaranteed of getting a piece of that 20 lot even though you were the first .50 bid. A scalper may get those 20 lots and lean on your 40 lots, hoping that you or someone else pays .75 before another seller emerges. On the screen the scalper has no like opportunity. If this order was attempted on Globex as soon as you "made the market" on your .50 bid you would be cued in first and any .50's that trade would be yours until the balance of your bid is satisfied. Not so in open outcry.
Another common situation. Lets say that 5 different brokers in different corners of the pit are offering 10 contracts each at .00 Your broker gets an order on your behalf to pay even for 50 contracts. As soon as your broker starts lifting the second or third 10 lot for sale, a smart local who senses that your order is still not satisfied, will quickly lift any other broker still offering evens. Thus you may only get 30 contracts even though in reality there were 50 offered when your order was beginning to be executed. Rules at the CBOT and CME differ. At the CBOT there is less "stealing of markets" because the broker may bid even for 50 and no one can buy the evens until your order is filled. There is no formal "rule" on this, it is called "pit etiquette", but it's somewhat enforced, accept back when bonds were at their peak and over 600 people stood in the pit. Back then "stealing markets" was the ticket to riches. At the Merc however, you may not bid into the offer. You may only trade individually with those who are offered. Thus your broker must say to each 10 lot offer "buy em", "buy em", "buy em", well after about three of those buy ems ain't gonna be any left. Now your broker must turn the market bid on your balance and once again a happy local is leaning on thr remainder of your order.