manage_edit-{post_type}_sortable_columns sorts, but wrong!

Remove this:

// Administration: Teach wordpress to make the column sortable
    function anco_project_year_column_orderby( $vars ) {
            if ( isset( $vars['orderby'] ) && 'anco_project_year_from' == $vars['orderby'] ) {
                    $vars = array_merge( $vars, array(
                            'meta_key' => 'anco_project_year_from',
                            'orderby' => 'meta_value_num'
                    ) );
            } else if ( isset( $vars['orderby'] ) && 'anco_project_year_to' == $vars['orderby'] ) {
                    $vars = array_merge( $vars, array(
                            'meta_key' => 'anco_project_year_to',
                            'orderby' => 'meta_value_num'
                    ) );
            }
            return $vars;
    }
    add_filter( 'request', 'anco_project_year_column_orderby' );

and add this:

add_action( 'pre_get_posts', 'my_slice_orderby' );
function my_slice_orderby( $query ) {
    if( ! is_admin() )
        return;

    $orderby = $query->get( 'orderby');

    if( 'anco_project_year_from' == $orderby ) {
        $query->set('meta_key','anco_project_year_from');
        $query->set('orderby','meta_value_num');
    } elseif('anco_project_year_to' == $orderby) {
        $query->set('meta_key','anco_project_year_to');
        $query->set('orderby','meta_value_num');
    }
}