The board of trustees annually shall allow interest at the rate determined by the board on the mean amount for the preceding year in each of the funds with the exception of the expense fund. The amount so allowed shall be due and payable to the funds and shall be annually credited thereto by the board of trustees from interest and other earnings on the moneys and other assets of the retirement system. From the interest allocated to the members’ savings fund, the board of trustees shall annually credit each member’s individual account with interest on the largest balance remaining in each account for the entire year and at the rate determined by the board.