deathmessage "Was killed by <playername>" when that player is killed by a mob

I first saw it in 1.5.0, when a skeleton shot me. But the weird thing is: sometimes it shows the RIGHT message, and sometimes, it doesn`t. The skeleton that shot me was customized to drop custom armor. In earlyer versions, it said:"<playername> was shot by skeleton". But now it (sometimes) sais:"was killed by <playername>". When I came back, I killed him and saw he dropped a custom bow. I`m not sure, but I think custom items are causing the problem. I don`t remember naturaly spawned skeletons causing this problem!