In this question you have mentioned endpoint, that means the whole reaction has taken place . For complete reaction the reactant must not be left, only the product should be there. So complete neutralisation is counted for the whole.
Whereas, The initial volume of base is given and total utilised base is also given. by formula the eq of acid = eq of base we can assume total used acid(HA) in the solution as 26.6 ml. So we take all the required acid at the begning of the experiment and what remained un-utilised is left at final stage (12.6ml left here).