Work type is shockingly complex for something that seems so simple. In the upstream data sources that we use (datacite, crossref, hal, etc) there are different taxonomies for what work type means and very little effort applied to ensure that when records are created, the submitters are selecting the appropriate work type even in their native taxonomies. That makes things super complex.
We have been working on some enhancements to our algorithms on this (check this out). We have a few more enhancements we're hoping to complete in the next couple of months that will continue to improve these classifications. Once we have the next set of improvements to our process, the next step is to open up to community curation so that users like yourself can suggest changes when you see them. In the meantime, if you would like to suggest a change to the work type field for a particular work, you can do that using this google form.