The theory is that God allowed Samuel to come back as himself, not a ghost. Or it was a demon impersonating Samuel and the Witch of Endor was surprised because she saw a real spirit. But no ghosts. That's a translation issue.
This has been a really interesting thread...
I had always thought/was taught that the reason the Witch of Endor was surprised is because for whatever reason, when she brought up whatever it was she brought up, she then realized who it was that had asked her in the first place.
Before she ever conjured up (whatever it was she summoned), she had protested because Saul, the king of Israel, had forbidden all witchcraft in accordance with God's command. But the man who asked her said not to worry and to do it anyway--and this man, of course, was Saul in disguise. I'd always believed the reason she was surprised/said she had been tricked was not because of the result of her work, but because it was at that moment she realized it was Saul himself asking, and she was afraid she'd be put to death because of it.
But this could be another explanation. This is why I like reading what other people have been taught/concluded...
(Lil, I'm sorry I can't answer your question... I'm not sure if "ghost" can be translated another way.)