Determination of interest depends on how the pension scheme investments have performed during the year. Also depends if you are in a guaranteed fund i.e. the type offered by insurance companies for small pension schemes pooled together into one fund. The minimum guaranteed rate of interest is 4% irrespective of how bad the insurance has done on the investment-risk is borne by insurance co. Most insurance companies give interest of 6-12%. The rate is determined by the insurance company.
If the fund is segregated i.e. for the big pension schemes then the trustees and members take the risk of investment if market is good the higher the return. Usually the fund manager report on the fund performance audited accounts are done and the trustees decide the interest to be credited to members accounts.
All in all interest rate depends on how well the fund managers perform. Does that help?

@spgizzi, it's been a while since i looked at pension accounts. However from my memory bank, the crediting of member accounts with interest depends also on the size of the pension scheme. In smaller schemes the sponsor company meets the RBA levy, trustee expenses, management fee and audit fees allowing the members to get a higher interest i.e. the rate announced by the insurance company. If the scheme sponsor does not meet the expenses then before the interest is credited to members account the expenses are deducted from the investment income and the net interest is what is credited. RBA has a limit on the expenses as a % of the fund.

In segregated funds you can find members having a negative return in some years as the pension scheme expenses are offset from the investment income.