Bayrita is a white clay that was under the transparent gelatin-silver emulsion on so called "fiber base" papers, between the emulsion and the paper base itself. I think most, if not all, FB pares still have that baryta.
"RC" papers have the *same* emulsion, but baryta does not adhere well to the polyethylene coated paper "core" (coated on both sides) - so titanium oxide (same pigment as used in oil paints) is used. Titanium oxide - if memory serves - is "whiter" than baryta.
I think the difference in appearance is probably due to either a reduction in the thickness of the emulsion, less silver in the emulsion, or a combination of the two.