The approximate and actual contributions are similar to premiums under insurance policies. The approximate contribution quoted by the board to the public agency is subject to the contingency that the actual contribution certified by the board after the approval of a contract may differ from the approximate contribution because of:
(a) Change in number or salaries of employees included.
(b) Change in prior service benefits.
(c) Time elapsed between the quotation and effective date of the contract.
(d) Change in effective date of membership.
(e) Change in manner of determining contributions.
(f) Any changes in the facts or assumptions upon which the quotation was based.