Guilin Sun
Ansys Employee

It is in this page https://optics.ansys.com/hc/en-us/articles/360034382614

This method is suggested to be used for photonic inverse design, not for every simulation, as it uses some kind of average method.

I strongly suggest you to use conformal meshing methods:

https://optics.ansys.com/hc/en-us/articles/360034382614-Selecting-the-best-mesh-refinement-option-in-the-FDTD-simulation-object

To reduece the difference, you may need to use finer mesh.