Isn't the issue less that there is an average but that there was a guaranteed return? The PERS guide says that prior to 2003, higher earnings were passed through to members. So then you have  a system where earnings might be 16%,0%,16%,0% which would have an average earnings of 8% but a average deficit of 4%. Giving it all in the good years and covering the bad years is a sure way to rapid ruin.