HFSS and CST/MWStudio are equally good tools for simulating WG filters.
MWS has a choice of several solvers. Their FD solver is good for filters , their transient solver is not so good , it is more suited to less resonant but broadband structures like antennas and couplers, but it is still usable and useful as final check .
It sounds like you are doing a check anyway between HFSS and MWS and are getting close agreement but in reality you are just over 1.5% off in your centre frequency.
For a filter that's not too bad. They can be very sensitive to machining tolerances, or imperfections in soldered joints etc, -which is why tuning screws are often needed. Thermal expansion can be another problem for the same reason. You might want to calculate what machining inaccuracy will produce the error you are measuring.
If you are slightly too high in frequency adding a tuning screw in the cavity should fix the problem.