I have asked Select this question 2 years ago & their production manager Tony's answer was when they go to buy the cardboard to print the cards on the supplier can't supply enough for the print run so they have to get the balance required from another supplier & there is shade variations & that's why there is different shades of colour on the cards.
But you would think the variation would exist on all cards in the pack & box, but this not so it's usually on one or two cards.