原因是内部有此代码dbDelta() :

// Create a tablename index for an array ($cqueries) of queries.
foreach ( $queries as $qry ) {
    if ( preg_match( \'|CREATE TABLE ([^ ]*)|\', $qry, $matches ) ) {
        $cqueries[ trim( $matches[1], \'`\' ) ] = $qry;
        $for_update[ $matches[1] ]            = \'Created table \' . $matches[1];
    } elseif ( preg_match( \'|CREATE DATABASE ([^ ]*)|\', $qry, $matches ) ) {
        array_unshift( $cqueries, $qry );
    } elseif ( preg_match( \'|INSERT INTO ([^ ]*)|\', $qry, $matches ) ) {
        $iqueries[] = $qry;
    } elseif ( preg_match( \'|UPDATE ([^ ]*)|\', $qry, $matches ) ) {
        $iqueries[] = $qry;
    } else {
        // Unrecognized query type.
    }
}