My guess is they are sending out gibberish but eventually after the South Koreans have grown tired of trying to figure it out, they will send out the real one.
That is why spies on the ground are still necessary. That way the South Koreans will know what is gibberish and what is not.
That is what I suspect too. It is relatively cheap for NK to operate the transmitter. Meanwhile, SK and Japan and the US will expend brainpower and compute time trying to figure it out. Might even be aimed at flushing out our agents in the north. Transmit about something fairly unique, then wait and see who turns up asking about it. Hope our guys are smarter and more subtle than that!