If you had a 10% return one year and a 90% return the next, for an average of 50%, your total principal would increase by 1.1 * 1.9 = 2.09. If you calculate by

If you had a 10% return one year and a 90% return the next, for an average of 50%, your total principal would increase by 1.1 * 1.9 = 2.09. If you calculate by the average instead (1.5 * 1.5 = 2.25) the rate of return is higher, and that error may have compounded in your calculation.