Enthalpy - unit confusion
25.9 kJ/mol refers to one mole of $\ce{HI}$ produced while 51.8 kJ/mol corresponds to one mole of $\ce{H2}$ consumed. It is good practice to give the full equation, especially when dealing with reaction enthalpies/energies $\Delta_rX$. For formation quantities $\Delta_fX$, I would always assume one mole of product.