As I can see from the model, you are using vacuum and PEC as the only materials for the objects. HFSS will also enclose all the objects with PEC. This means that there are no losses in your model and all of the energy that isn't stored in the cavity will be reflected back. Also, your cavity doesn't have any resistive properties, which in turn makes the Q of the cavity very very large, meaning the resonances will be very sharp and with an extremely narrow BW.
The results you are observing are most likely due to the interpolating sweep's accuracy. You can try increasing the Sweep points, as well as the Error Tolerance of the Interpolation, in the Interpolation tab of the Sweep.
If you'd like HFSS to solve for every single point of your sweep, you can try the Discrete Sweep as an option that will also give extremely accurate results.
As a suggestion, you can always plot the VSWR and see the very sharp resonacnes.
Finally, try using an actual conductive material, like copper, for your cavity, you will be able to observe all of it's properties very clearly.
Hope this helps!