Looking at the series 1 and 2 cards on the linked site.
To me it looks like the ones discounted are a matter of plain backgrounds being removed. A close up should show some degree of halftone on the dark ones and none on the others.
The ones where there are different colors for the "clean" backgrounds are less clear. Colors were often mixed by hand according to a recipe. Those could be sort of imprecise, so you'd get different colors for different batches. Especially if it was a different press operator.
Thos differences could be different runs, like one guy ran a batch, then a different guy ran a batch printed a month later.
It's probably impossible to know if there were different actual runs or sets of plates and what was done when or if it was just the guy on the next shift mixing the ink differently.
I would keep them if I got them, but probably not actively go looking for them.
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