The obvious difference (to me, anyway) is:
- Arya scried Eragon, vs
- Arya scried on a small pool of water the battle under Farthen Dûr.
Scrying usually involves some surface on which the image of the object being scried appears. When you scry something, the image of that something appears. When you scry on something, the image appears on that something.
Of course, this seems to have been intended to be humorous. Bennett says:
The notion that copyediting is an often thankless task was belied—or
better, blown out of the water—by Christopher’s boundless generosity.
When he graciously acknowledged me in Eragon as “the only man alive
who understood the difference between ‘to scry it’ and ‘to scry on
it,’ I had to quickly learn the difference.