Why would that be important to a ghost?
Indeed, I'd venture to say that you were simply lucky.
I would also be willing to say that it almost certainly was a dream. You were 9, and an admittedly heavy sleeper. The chances that you were suddenly awoken by anything is slim. Anything except a bad dream, that is.
You've also had 20 years of retrospective memory to make your memory match what you think happened. It needn't be anything you're even consciously aware of; we all tend to make our memories conform to the version of events we are convinced occurred.
For example, suppose that you were awoken by someone (grandma it seems) entering the room and turning on a light? There is a distinct possibility you were already dreaming about your grandfather just prior to that, or that you dreamt about him immediately after going back to sleep. The next morning this could have been merged into a single event.
You may even have seen someone in those waking moments who wasn't your grandfather, but who looked like him because a) it was dark, b) you had just woken up, and c) there was an inopportune light shining towards you.
That, or you saw a ghost which happened to look like he ought to just for your sake, who somehow managed to wear clothes, and who thought not to communicate a final goodbye to his wife and children, but to simply create alarm for his 9 year-old grandson.