I am fairly certain back then the cardbacks were printed using offset printing. Essentially the process uses 4 images printed on top of each other each one in cyan, magenta, yellow and black.
The alignment of the 4 images must be precisely laid on top of each other in order to yield a sharp image but if one or more images are slightly off, the effect you see is what can occur.
Generally the printer has a small mechanical issue that needed tending to -or- some external vibration occurred when one or more colors were being applied -or- just some other anomaly. I believe the term used in offset printing is mis-registration for this kind of printing error. My printing expertise is based on other technologies and I just have a basic understanding of offset so that's the best I can offer you there. I know dye-sublimation, inkjet, lasers and projection/lens based printers but any printing that doesn't lay down the image in one shot is susceptible to this type of mis-alignment.
Anyway............. since yours isn't too bad looking, it just cruised past QC at the printer. If it was way off, it would most likely fail QC.
There are a few other examples that are shown in the Palitoy 45 back Toy Toni threads.
Hope that helps a bit.