为什么$ stmt-> num_rows在MySQL LIMIT之前计算？

I'm executing a SQL Statement in PHP, using LIMIT $start, $offset to paginate results.

An unexpected hurdle, is $stmt->num_rows is returning all rows regardless of SQL LIMIT. Here is my code, just incase I haven't made a silly mistake:

$term = '%'.$search.'%'; // I have used htmlspecialchars() above
    $reclimit = 20;
    $start = ($page - 1) * $reclimit;
    $offset = ($page + 2) * $reclimit; // I want to get 60 results in total, or 3 pages of results.

    $mysqli = new mysqli("localhost","username","password","db");

    $stmt = $mysqli->prepare("SELECT title,description FROM content WHERE title LIKE ? LIMIT $start, $offset");

    $stmt->bind_param('s', $term);
    $stmt->execute();

    $stmt->store_result();
    $stmt->bind_result($title, $desc);

    $rowcount = $stmt->num_rows; // This will get all results, regardless of `LIMIT`
    $tpages = ceil($rowcount / $reclimit); // $rowcount returning incorrect integer messes up my pagination

    echo $start;
    echo $offset;
    echo $rowcount;
    echo $tpages; // Testing all variables to see which was the culprit

    $wcount = 0;
    $wmax = 20;

    while(($stmt->fetch()) and ($wcount < $wmax))
    {

        // Echoing HTML --snipped--

        $wcount++;
    }

    $stmt->close();

    $mysqli->close();
    }

I can't see anything I've done incorrectly, and it seems strange $rowcount would return a value from all returned rows, regardless of LIMIT as the data returned has been limited before the sum has been called. Have I misunderstood something?

Thanks for your time