Because \( 1200*\require{enclose}\ddot{a}_{\enclose{actuarial}{3}}^{(12)} \) is present value at 1st Jan, 2015 but you need at a year before.
If you think to apply \( 1200*\require{enclose}a_{\enclose{actuarial}{3}}^{(12)} \) , then its wrong. Because, it means present value of payments for 31/3/14 ,30/6/14 ,30/9/14 so on.
Last edited: Jan 14, 2015