'Dispersion' can have several meanings depending of the context.  In that particular case, I think that 'dispersive reflectivity' refers to the dispersion (vs wavelength) of the phase change under reflection, i.e. the phase of the complex reflection coefficient of the Bragg gratings.  From this phase dispersion, you can calculate the chromatic dispersion and Group-Delay Dispersion, which particularly affects the shape of short pulses reflectance by the Bragg gratings.

I hope that this helps.

@ Daniel Poitras sir, thank you so much for your answer.