What a great find! I guess this has to do with Clomps being introduced (10.90) after the achievement (8.70). It could be a bug, and I suspect other creatures introduced after 8.70 may be affected as well. (High Lizards?)
Why do I think it may be a bug?
Because there was a bug with skinning Bonebeasts, I suspect the "skinning successful" counter is incremented per defined use-with action pair (Obsidian_knife, Bonebeast_body). In that bug, the counter was inadvertently incremented also on failures. In the case of Clomps, I believe such change to the counter is missing.