As I have already said the explanation as to how this happened may never be revealed. I can think of some possibilities though. All involve a visit from the Cock-up Pixie:
The stamp was not originally intended as a Journal freebie, but a regular stamp. Then a sport on the sheet would be normal. This is unlikely because the sheet has no margin decorations which would be normal procedure. In fact there is insufficient margin for that as the sheet was prepared for maximum number of stamps per sheet. So when the sheet make-up was changed the sport would not be added (surely?).
A sport was required but would have been removed and put aside before the stamps were put into the Journals. This could then be a special prize stamp. Against this though, is that someone would have had to forget all that between briefing and delivery of the stamps.
Sports were becoming so commonplace that it was assumed that one could and should be added. Were the design team aware that sports were not for the Journal stamps?

I know which I favour, but it is only a hunch so I leave you to decide for yourselves. Whatever the cause, these mistakes are so easy to make. I am sure everyone has examples from work where a simple but important message has not been passed on.