Upon retirement, the maximum retirement allowance of any member whose accumulated contributions are deficient shall be reduced by an amount which is the actuarial equivalent of the amount of the deficiency. A deficiency shall be the amount by which a member’s accumulated contributions fail to equal the accumulated contributions which would be standing to the member’s account had the member contributed at the full rate required by law. Deficiencies may be paid by the member in advance of retirement, in which case the member’s retirement allowance shall not be reduced.