It's possible it was simply a case of someone at USPC grabbing whatever design was available. They needed to churn out some calendars, they noticed there was a back design that wasn't in use, decided "what the heck, it's paid for" and used it.
Just a guess.
For a more modern example of something similar, look at the Bicycle Dragon Back decks. They were advertised as being a long-lost back design, thought to have never been printed, and were "colorized" for their Bicycle release a handful of years ago. In fact, it was an old Tally Ho design - I've seen it for sale on eBay as a single swap card. Someone found it in the archives, decided "what the heck, it's paid for" and used it!