If I use mean variance optimisation to create an efficient portfolio with a target expected return of 20% in a year's time and find that the actual return at the end of the year was 24%, what explains the deviation from the target? Is it just a matter of rebalancing?
If you form a portfolio at time $t$, in which the weights are chosen to get an expected return of 20%, you will certainly not get exactly 20% at $t+1$. If that was the case, you would not bear any risk.
What you do is that you form a portfolio that will get a 20% return in expectation (on average if you want), you may end-up with more or less than that in the end.
This is just a matter of expected value and realized value of a random variable.