That's what I've always thought as well. Here's a link that explains hypnopompic:
http://en.wikipedia.org/wiki/Hypnopompic
If I'm understanding correctly (in my regurgitated interpretation), hypnagogia is a more "rational" cognizant state. Even though hallucinations occur, our perception is adequate enough to process it, and think more clearly. Where a hypnopompic state would be where we are just flat-out confused; talking jibberish, etc. Which then makes me think that when people are said to wake out of sleep and
supposedly "speak ancient tongues" - it would be a hypnopompic state; they aren't speaking ancient tongues, they're just confused beyond belief.
So maybe that's why hypnagogia is associated with going into sleep - because we haven't made it to a deep sleep yet and haven't completely lost the "awareness". Once in a deep sleep, our brains are off and away to nowhere zone, and therefore harder to think "rationally".
How's that for a pure scientific definition
That's how I'm making sense of the difference. Please, correct me if I'm misunderstanding.