Based on the context provided, there could be several reasons for the discrepancy you're observing between your Ansys and Matlab results. One possible reason could be the difference in the damping models used in Ansys and Matlab. In Ansys, the damping ratio you specified is used in the modal superposition method for transient analysis. This method transforms the governing equation of motion into an alternate form using modal coordinates and a linear combination of mode shapes. It is more efficient than the full solution method as it converts the coupled equation of motion into modal form. Another possible reason could be related to the 'Missing Mass' method in Ansys. If you have not extracted or included enough modes in your analysis, the 'Missing Mass' effect tries to include the effect of these modes in an additional pseudo-static mode shape. This could potentially lead to a more damped system in Ansys compared to Matlab.