mode analysis for 2D slab waveguide

Hi, Everyone,

I am new for using comsol, and firstly I built a easy model to learn.
It is a 2D slab waveguide (see attached ).

By mode analysis, I found the effective mode index obtained by comsol is different with the value obtained by analytical solution.
In comsol mode, the infinite direction is set as PBC with continuity.

I appreciate you can give me some help.

Regards
Marvin