I don't believe the Statistics menu would register these kills as mine unless I did kill them.
Skeletons killed all by hand as per this video (https://www.youtube.com/watch?v=1e4OH5AiHfY), skim through if you wish. I believe skulls are also dropped if killed by a tamed wolf, however that's not part of the issue.
I don't believe the Statistics menu would register these kills as mine unless I did kill them.