looting from different fishes makes the objects going into a different stack

I have 2 theories about it. It may be because slaughterfishes and small slaughterfishes are different kinds of monsters or, because when I looted slaughterfishes, it was a crime, but not for small slaughterfishes. So, on some objects, an owner is reported, but not on others, which could make the items "different".

(RM-1212 from redmine: created on 2014-03-15 by Bérenger Morel, , closed on 2014-03-16 by nobody, duplicates #1100 (closed))