If you get 8% returns per year on average, the expected annualized return over a multi-year period is going to be LESS than 8%.
This is because you won't get exactly 8% each year. For example, suppose the returns over a 3-year period are: 20%, -10%, 14%. In this case, the return over the whole period is 23.21% (= (1 + 0.20) * (1 - 0.10) * (1 + 0.14) - 1). On the other hand, a 8% return each year would have resulted in a 25.97% return over the whole period (= (1 + 0.08)^3 - 1).
