stackrt is analytical solution so it is accurate.
Simulation results depend on the simulation settings. Smaller gid size does not necessarily mean higher accuracy. When you used finer mesh, did you increase the PML layers to keep about the same thickness? if not, thin PML may not perform well and may reflect to reduce the net transmission.
Covergence testing in FDTD involves several setting adjustments. Please refer:
Convergence testing process for FDTD simulations
Please note that simulation from discrerized mesh will introduce discrete errors. Similar to experiments with measurement errors and setup errors as well as manufacturing errors, we can control them to certain level, but cannot eliminate them. If the simulation is accurate to certain decimals, it should be ok.
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