Discrepancy in elastic constant calculation

Dear LAMMPS users,

For my ceramic system, when I calculate Elastic constants using the proposed code in 0 temperature in LAMMPS examples, I find that all of the elastic components except C33 are in good agreement with Experimental data. Any probable explanation for it please? For a better understanding, C33 should be 500GPa. But I obtained it as 2345GPa !!!