Basically as i see it, its really just a plot hole..There really isnt a lot of rhyme or reason behind these things sometimes, especially because this is simple backstory not really shown as far as im aware in any canon material. To add to that thought i cannot even confirm 100% that that information came from the author himself. That being said if i had to put a theory out there (not backed up by any sources mind you) id say that perhaps if the person whos been killed has just died the soul still hasnt left the body and therefore can be brought back, however even then its a little iffy because deus and murmur are described as "godlike" however it is clearly stated that neither of them are actually gods persay. Hope that helps even if there isnt a absolute answer.
TL;DR
PLOT HOLES!