I'm having a little trouble understanding your questions but let me try to clarify some things for you. An annual percentage rate is NOT the same thing as an effective annual rate (EAR). The concept of annual percentage rate, while helpful in comparing rates, has limitations. Generally, margin lenders apply the stated annual rate to each monthly statement period by dividing said rate by 360 and then multiplying by the number of days in the period. The resulting rate is then applied to the account's average daily debit for the period. Further, I am not aware how widespread the use of the 360 day convention currently is.
My reference to compounding is an attempt the convey the fact that each month interest will be added to the debit balance. If this interest is not paid interest will apply to the prior month's interest in the next statement period. As you can see, the EAR after 12 months will not be the same as simply applying the APR to the initial debit balance.