Chapter 10, Problem 79E

Hydrogen, a potential future fuel, can be produced from carbon (from coal) and steam by this reaction
$\text{C}\left(\text{s}\right){\text{+2H}}_{\text{2}}\text{O}\left(\text{g}\right)\to {\text{2H}}_{\text{2}}\left(\text{g}\right){\text{+CO}}_{\text{2}}\left(\text{g}\right)$

Use average bond energies to calculate $\Delta {\text{H}}_{\text{rxn}}$ for the reaction, and then use standard enthalpies to calculate $\Delta {\text{H}}_{\text{rxn}}$ . Why are the two values different and which value is more accurate?