They assume that the shares will come in and "act" like they have them already. Just like when you deposit money, usually it takes days for the transaction to occur but your bank will assume that the money is real and you can use the mo ey sooner than the actual time ot takes to process.
Also the same as when you make a deposit into a trading account. Some institutions will let you use the money to place trades right away even though they haven't collected from your bank yet. They are in essence floating you the deposit money under the assumption that your deposit will come through within days.
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