In the Lammps example on how to calculate thermal conductivity, Example 2 of the ReadMe file goes like this
Any number of things could have changed in the code in the ~2 years since
that README was written. These are small problems, run quickly, for demo
purposes. The difference between 0.71 and 0.78 is not significant.
If I run it today, I get the same answer as you.
Steve