Could be the ones with unresolved baggage that come back. Or maybe there's some weird... cycle to it? Heck if I know. The afterlife is already a weird enough concept before you start factoring in reincarnation.

[Which draws Riku's attention to the various Hex Maniacs and others scattered around the area.]

Honestly, I'm surprised these guys don't round up more ghost Pokemon for their research. But maybe the fact that they're doing this at all proves there's more going on with death.