Iginla2010

You calculate the rate of return by comparing the ending value after investment return to the beginning value available for investment.
Thus, if you add money at the beginning of the period, it’s available for investment, so you include that money in the original amount to compute the return.  If you add money at the end of the period, it is not the result of investment return, so you don’t include it in the ending amount to compute the return.