rss widget displaying wrong categories

here’s my steup:
I have a the default rsswidget displaying posts from my blogpage (http//:mypage.com/blog),

I’ve excluded some category from blog page with this function:

function exclude_category_home( $wp_query ) {

    $excluded = array(20,36,37,1);
    if( is_home() ) {
        $wp_query->set('category__not_in', $excluded);
    }
}

add_action( 'pre_get_posts', 'exclude_category_home' );

but my rss-widget still displays the categories I’ve excluded,

anybody can help?