Then it has to be different from OM. So the explanation Kartikeya gave was about the secret Mantra. This makes sense because OM was known to others & is part of many Mantras. What others don't know should be kept a secret, that's why it was explained in Shivji's ears as mentioned by you. It's good to know the proper version to get a better idea then just knowing a part which may creat confusion.Actually what Murugan propounded was not just about "om" - but it was the secret mantra which was used by Brahma for creation. The mantra which breathes life into matter- the mantra which makes a creation come to life. That's why Skanda said it could not be uttered frivolously.