These could have been end of line packaging before palitoy went belly up, when they reached the end of line they would probably had loads of left cards and accesories from earlier lines and just packaged with what they had left.
the thing is that the two that i have are both on 2nd Series cards w/ the offer for the Commander figure on the back, not the Skeletron. that is why i think this seems to be so odd. your idea is the easiest to understand but the use of 2nd series cards is the weird part. of course they could have had an overstock of these cardbacks as well.