Although the experiment that brought Darryl back to life appeared to be purely scientific, it is likely that the Hellmouth was able to exert some influence over the experiment to ensure its success, much like how it indirectly contributed to Marcie Ross's state of invisibility by responding to Marcie's own perception of herself.
Considering Darryl's obviously warped psychological state, where he's willing to kill another girl to have a companion, it creates the possibility that the process that brought him back to life failed to restore his soul at the same time.
