The acid is most likely a diprotic acid which is able to donate two $H^+$ ions. Therefore, it will take twice the amount of a strong base such as NaOH to completely react. Similarly, the strong base is likely able to accept to protons. Therefore, we only need half the amount of this strong base to completely react with HCl.
Work Step by Step
A diprotic acid is an acid that donates two $H^+$ ions. Therefore, if the strong base is only able to accept one $H^+$ ion, then we need twice the amount of base to completely react. Similarly, some bases are able to accept two $H^+$ ions from acids. Therefore, because $HCl$ only donates one $H^+$ ion, we need only half the amount of base.