为什么mysqli_fetch_all（）不能与一个结果行一起使用？

Question

I'm confused about the results I get from using mysqli_fetch_all() and mysqli_fetch_array.

I wrote a function to grab arrays from the primary key table that match the foreign key of another table where $field is the desired field in the primary key table and $property in the FK table.

function getProp($table,$field,$property,$cnn){
    $findRow = "SELECT * FROM $table WHERE $field='$property'";
    $res = mysqli_query($cnn,$findRow);
    if ($res) {
        $row = mysqli_fetch_array($res,MYSQLI_ASSOC);
        return $row;    
    } else {
        echo "we couldn't find the row that matched $field";
    }
}

Now the idea is that there should be only one match found in the primary key table. So I thought replacing mysql_fetch_array() with mysqli_fetch_all() would be okay. However, when I invoke the function like so:

$profileRow = getProp('profile','id', $runner_id, $cnn);
$profileRow['question_text'];
$profileRow['avatar']`

I get the following error:

Undefined index: avatar

I would appreciate any help. I looked at the PHP manual, but don't understand why these MySQL actions yield different results in my case.

EDIT--

Thanks for your help, but I was specifically asking about what caused the error. Please explain to me as if I were a six year old - most technical explanations pass over my head. For example what does this mean?:

mysqli_fetch_all() should only be used in those situations where the fetched result set will be sent to another layer for processing.