在boddie，投资学第八版中，147页。Forecast for the long haul 从长远来看预测

Sampling errors in the estimate of expected return will have asymmetric impact when compounded over long periods.
Positive sampling variation will compound to greater upward errors than negative variation.

英文字面意思我能理解。请达人讲一下这两句话内在的意思？
尤其是不理解为什么这里会出现复利计算（compounded）?