在…内get_posts() 属于WP_Query 类，您将看到此代码块。

/*
 * Ensure that \'taxonomy\', \'term\', \'term_id\', \'cat\', and
 * \'category_name\' vars are set for backward compatibility.
 */
if ( ! empty( $this->tax_query->queried_terms ) ) {

    /*
     * Set \'taxonomy\', \'term\', and \'term_id\' to the
     * first taxonomy other than \'post_tag\' or \'category\'.
     */
    if ( ! isset( $q[\'taxonomy\'] ) ) {
        foreach ( $this->tax_query->queried_terms as $queried_taxonomy => $queried_items ) {
            if ( empty( $queried_items[\'terms\'][0] ) ) {
                continue;
            }

            if ( ! in_array( $queried_taxonomy, array( \'category\', \'post_tag\' ) ) ) {
                $q[\'taxonomy\'] = $queried_taxonomy;

                if ( \'slug\' === $queried_items[\'field\'] ) {
                    $q[\'term\'] = $queried_items[\'terms\'][0];
                } else {
                    $q[\'term_id\'] = $queried_items[\'terms\'][0];
                }

                // Take the first one we find.
                break;
            }
        }
    }

    // \'cat\', \'category_name\', \'tag_id\'
    foreach ( $this->tax_query->queried_terms as $queried_taxonomy => $queried_items ) {
        if ( empty( $queried_items[\'terms\'][0] ) ) {
            continue;
        }

        if ( \'category\' === $queried_taxonomy ) {
            $the_cat = get_term_by( $queried_items[\'field\'], $queried_items[\'terms\'][0], \'category\' );
            if ( $the_cat ) {
                $this->set( \'cat\', $the_cat->term_id );
                $this->set( \'category_name\', $the_cat->slug );
            }
            unset( $the_cat );
        }

        if ( \'post_tag\' === $queried_taxonomy ) {
            $the_tag = get_term_by( $queried_items[\'field\'], $queried_items[\'terms\'][0], \'post_tag\' );
            if ( $the_tag ) {
                $this->set( \'tag_id\', $the_tag->term_id );
            }
            unset( $the_tag );
        }
    }
}

正如你在第二个foreach 回路，用于category 在分类法中，WordPress仅使用第一个查询的术语从中提取slug，然后将其设置为query_vars. 同样的情况也发生在post_tag.

WordPress为什么这么做，评论已经说明了一切。