Extending WP_List_Table seems getting wrong with $this->get_columns() – what’s wrong?

This,

$_wp_column_headers[$screen->id]=$columns;

…is deprecated long ago. Instead you need to assign your columns to the following class property on WP_List_Table,

$this->_column_headers = array( 
    $this->get_columns(), 
    array(), //hidden columns if applicable 
    $this->get_sortable_columns()
);