What Robin knows is different. It is one thing to know who the admirals,yonkou are and it is another thing to know about One Piece itself.
Truthfully I forgot about the ancient weapons,yet again I still see it as different. The weapons are unwanted unneeded knowledge. The SH don't need them right now and no reason to talk about them. Meanwhile Yamato knows things about the Journey in itself and presumably the conclusion that Rayleigh told us ,that Roger pirates reached about what happened in the void century..
Robin and Yamato on the same ship ,yet to do not talk about these important things? A no go . Seems weird.
About the D in itself Robin doesn't know much as she told Law,even in this subject Yamato knows stuff due the Log she has read extensively .