We were talking about this situation at work today.
It seems on ocassion, there are times especially when younger, where you remember something differently than how your parent or an adult remembers it happening. That's pretty plausible. But usually it's something like getting your arm broken playing football or something like that.
In this case, getting shot at with an RPG is a pretty traumatic event in a guy's life. With these sorts of things, you don't just forget how it happened.
He's just a liar is all.