Football season cards are printed much later in the year than baseball. Junk era baseball sets typically hit late the previous year ie, Nov/Dec '90 release for 1991 baseball sets while 1990 football sets have been out a few months. They would be printed at the same the time or would overlap one another for a period.
Those 1991 Topps BB/1990 Topps FB cards are very common, at least a decade ago you'd see singles and sheets all over ebay. Not surprised that Fleer stuff turned up. 1991 UD Baseball and Hockey ones have also turned up in smaller quantities.