Higher silver content.

Papers certainly differ in Dmax, going from 1.6 (matt papers, e.g. Fomatone 312) up to 2.2 or more with some glossy bromide papers. Not saying because of silver though.

I believe that would also be due to light scattering on the surface of the matt papers, increasing the amount of non-image light randomly reflecting off the surface...which dulls the appearance of the blacks a little (which can be a good thing).

That's because all papers have more or less the same DMax and one being silver rich than the other makes no real sense.

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It will probably affect toning (and other playful things to do to prints)...but so will many factors.

Vaughn said:

I believe that would also be due to light scattering on the surface of the matt papers,

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Correct. It has nothing to do with silver content.
Ferrotype a matte print to make it glossy and the reflected dmax increases.