@Outinthegardener
Kei & Iori, um, no, never mind, while Kei got his after taking down a Dungeon Boss (or worse), Iori got hers from earlier in the dungeon.
The specific examples you list do support your theory.
Statistically far too small a sample size, but then again the current point of translation [end of volume 7] is indicating that statistics can hang, there is intent involved.
Random drops aren't.
There've been musings [in story as well as in these forums iirc] that the first to trigger orb drops for a given critter is the one who sets what orbs they have the potential to drop; things have been far too convenient otherwise. iirc Kei & Miyoshi realize it's been too convenient...