This behavior is often seen, with any method, when the grid size is too coarse to resolve the relevant wavelengths of the problem. In practice, you should make sure the ratio of element size to minimum wavelength (here \lambda= v_p/(3 Hz )=1000 m ) is <1.
The dominant frequency of your Ricker seems ~1 Hz, thus it has little power at 3 Hz, it’s dominated by wavelengths \approx 3000 m and thus better resolved.