Usually, IEEE conference proceeding articles having 'IEEE' at the bottom of each page in published articles. But some conference proceeding articles having only conference name and not having 'IEEE' at bottom of page after adding in IEEE Xplorer?
"... as the world's largest technical professional association, IEEE's membership has long been composed of engineers, scientists, and allied professionals. These include computer scientists, software developers, information technology professionals, physicists, medical doctors, and many others in addition to IEEE's electrical and electronics engineering core. For this reason the organization no longer goes by the full name, except on legal business documents, and is referred to simply as IEEE."
It indicates that IEEE is not technical part of those conferences. Simply stating IEEE conference does not make it IEEE conference, it should be approved IEEE and a valid record is maintained at their end.
I hope that is uploaded by the conference Organizers or Guest Editors only. Prior to Conference guest editor's has to take permission from the IEEE Editorial office to get the approval. After the completion of conference presentations, guest editors need to send all copy rights forms, ethical committee approval letters, green certificate and other supplementary materials to IEEE Editorial. Then they will be upload in IEEE Digital Library with DOI and unique IEEE ID. Currently that's why the same manuscript/paper/chapter is also included in IET Digital library.