Sensing schemes based on Rayleigh anomalies (RAs) in metal nanogratings exhibit an impressive bulk refractive-index sensitivity determined solely by the grating period. However, in this work, we demonstrate that the surface sensitivity (which is a key figure of merit for label-free chemical and biological sensing scenarios) cannot be attributable to the wavelength shift of the RAs, which are completely insensitive to local refractive-index changes but rather to a strictly connected plasmonic effect. Our analysis for increasing (up to wavelength-sized) overlay thickness reveals an ultimate surface sensitivity that approaches the RA bulk value (depending solely on the grating period), which turns out to be the upper limit of grating-assisted surface plasmon polariton sensitivities.