Because of settlement - that is that date the Order was executed plus 2 days.
That is why you can't cover immediately.
The other factor that could affect the daily short figure is very interesting. If a sale is being initiated by the holder of restricted 144 stock, even though the owner of those shares is technically a long, the sale is listed as a short sale because the actual certificates are not yet "clean" via the transfer agent.
FINRA states that in order to ascertain the true "open short" position one should look at the bi-monthly short report. It was also stated that any of the daily shorts which were not delivered within the prescribed time would definitely end up on the FTD list.
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