Yes, two separate plates. If you compare several of the same card, it is possible to slight variances in the alignment, especially the stitches on the baseball which can be a bit all over the place!
The Campos especially, if you compare different examples on ebay and google images, you can make out some where the two layers didn't quite line up, and the correct red star is slightly visible beneath the black overlay.
As for how this occurred, there is a lot of speculation, and I cannot offer much more than that, being not too knowledgeable on the actual process of printing these cards. I think it is safe to say that somehow, on the plate for the black devices, a star was inadvertently added, leading to the black overlay. It would make sense, then, that there are so few Campos black stars...an error like that, I think, would've been caught fairly quickly. A black star where there shouldn't have been any would've probably leaped out on a spot check, and the error would've been corrected.