You might want to check the server advanced property BuiltinAdminsAreServerAdmins - even though they are not shown as a Server Administrator in the Analysis Server Properties Security pane, they may be admins through this setting. As the following link mentions, "By default, members of the local Administrators group are also granted administrative rights in Analysis Server. Although the local group is not explicitly granted membership in the Analysis Services server administrator role, local administrators can create databases, add users and permissions, and perform any other task allowed to system administrators."

https://msdn.microsoft.com/en-us/library/ms174561(v=sql.120).aspx