"the writing is on the wall" vs. "the handwriting is on the wall"

Solution 1:

It's not unusual for idioms to get modified slightly as people use them, due to misunderstandings and the imperfections of memory. I doubt there's an intentional difference in connotation here; the speaker probably didn't even realize he or she was using a "non-standard" version of the saying.

Solution 2:

Just because you heard it once on the radio, doesn't mean it is a common phrase, rather than a misuse of a different common phrase. In this case, perhaps the speaker just messed it up?

Sadly, NPR has become a haven for mixed and mangled metaphors. I think this can happen when a person hears a phrase second or third hand, thinks it sounds educated, and doesn't look into its history or meaning very deeply. To be fair, I suppose it is also likely to happen when someone is put on the spot with a microphone in their face, and is forced to come up with deep thoughts on the spur of the moment. Nobody's perfect.

My personal favorite, which one hears on NPR a lot, is "800-pound Gorilla in the Room". This is a mashup of an old 800-pound Gorilla joke/proverb, and the common metaphorical phrase the elephant in the room. One could argue that it is in fact a new hybrid phrase indicating something with immense power that everyone is trying hard to ignore, but in fact every use I've heard has been in the context of the latter meaphor. Clearly the speaker got the term from context somewhere, but didn't exactly understand what they were saying. Once you actually grok both phrases, which mean different things and are both quite vivid, you aren't likely to mix them up.