for many brokers it is the time it takes to "settle" the transaction... for example if you bought today the 8th.. it takes 3 business days for the transaction to settle or clear... money was transfered and stock was recieved... so it is a done deal and you for sure own the stock on the 13th...then they usually take two days to transfer.. or more in some cases.
With the advent of realtime transactions this should not take place any longer but they still go by it...