Seeing those last two, I'd have to think Joe D is right. The shaded ones have the look of a dirty plate if they were typography for the backs, and a dry or improperly wiped stone if lithographed. The shading is right where "gunk" would collect on the plate, especially the areas of the s and e of series in the top pair. The lower pair shows the same thing in different ways on both cards.
I'm going with a wild guess, and guess that they're either all early series cards where they cut corners to get more made quickly, or last series cards where the wipers were worn, the plates dirty and they didn't bother cleaning the plate or getting a better wiper because they were almost done with the print run and knew there wouldn't be more from the same plate/stone. And I'm leaning way in the directon of the latter.
Steve B
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