Missing variable in post php javascript

I'm trying to get a php/javascript dynamically populated combo box working through mysql queries and to the most part it works fine, that is until I hit the submit button. The scripts do exactly what I want up to this point.

From what I can see the replacement code is left out of the $_POST array. I assume this is happening because once the form loads it lists what fields to expect or creates the $_post array on the spot, therefore by inserting some new code it perhaps ignores it? Just a guess but seems logical, if so, how would one work around this? This has plagued me for a month or so now.

javascript:

<script type="text/javascript">
    $(document).ready(subpos_selectbox_change);
    function subpos_selectbox_change(){
        $('#subpos').change(update_position_number);
    }
    function update_position_number(){
        var subpos=$('#subpos').attr('value');
        $.get('get_list.php?subpos='+subpos, show_posnumbers);
    }
    function show_posnumbers(ss){
        $('#position_number').html(ss);
    }
</script>

get_list.php

switch($_REQUEST['subpos']){

    case 'AO2';
        $teamQuery = "SELECT * FROM establishment_positions WHERE position_filled='0' AND active='1' AND position_acrynom='AO2'";
        break;

    case 'AO3';
        $teamQuery = "SELECT * FROM establishment_positions WHERE position_filled='0' AND active='1' AND position_acrynom='AO3'";
        break;

    case 'AO4';
        $teamQuery = "SELECT * FROM establishment_positions WHERE position_filled='0' AND active='1' AND position_acrynom='AO4'";
        break;

    case 'AO5';
        $teamQuery = "SELECT * FROM establishment_positions WHERE position_filled='0' AND active='1' AND position_acrynom='AO5'";
        break;

    case 'AO6';
        $teamQuery = "SELECT * FROM establishment_positions WHERE position_filled='0' AND active='1' AND position_acrynom='AO6'";
        break;

    case 'AO7';
        $teamQuery = "SELECT * FROM establishment_positions WHERE position_filled='0' AND active='1' AND position_acrynom='AO7'";
        break;

    case 'AO8';
        $teamQuery = "SELECT * FROM establishment_positions WHERE position_filled='0' AND active='1' AND position_acrynom='AO8'";
        break;

    case 'SO1';
        $teamQuery = "SELECT * FROM establishment_positions WHERE position_filled='0' AND active='1' AND position_acrynom='SO1'";
        break;

}

echo '<select name="pos_number" id="pos_number">';
$ss = null;
$result = mysql_query($teamQuery);
while($row = mysql_fetch_array($result))
{
    $ss.= '<option value=' . $row['position_id'] . '>' . $row['position_number'] . '</option>';

}
echo $ss;
echo '</select>';

..and here id the html code before the javascript goes to work:

<div class="newusr">
    <table>
        <form id="newusr" action="includes/insertuser.php" method="post">
            <tr><td>First Name: </td><td><input type="text" name="firstname"></td></tr>
            <tr><td>Middle Name: </td><td><input type="text" name="middlename"></td></tr>
            <tr><td>Last Name: </td><td><input type="text" name="lastname"></td></tr>
            <tr><td>Username: </td><td><input type="text" name="usr"></td></tr>
            <tr><td>sex:</td><td><select name="sex"><option value="1">Male</option><option value="0">Female</option></select></td></tr>
            <tr><td>Position: </td><td><select name="subpos" id="subpos"><?php echo $subpos; ?></select><td></tr>
            <tr><td>Position No: </td><td id="position_number"><select name="pos_number"><option value="">choose a Position first</option></select></td></tr>
            <tr><td>Contractor: </td><td><input type="checkbox" name="contractor" value="1"></td></tr>
            <tr><td>Start date: </td><td><input type="date" name="empStrtDte"></td></tr>
            <tr><td>Active: </td><td><input type="checkbox" name="active" value="1"></td></tr>
            <tr><td><?php echo '<input type="submit"><input type="reset" value="Reset"></td></tr>'; ?>
        </form>
    </table>
</div>

HTML code after I change a value in subpos (from Chrome):

<select name="subpos" id="subpos">
    <option value="AO2">AO2</option>
    <option value="AO3">AO3</option>
    <option value="AO4">AO4</option>
    ...
</select></td>
<select name="pos_number" id="pos_number">
     <option value="2">805620121</option>
     <option value="10">805678998</option>
</select>

All looks good, does exactly what I want at this point. Then I print the array and boom, missing the pos_number value and I have no idea why.

Array
(
    [firstname] => test
    [middlename] => test
    [lastname] => test
    [usr] => 
    [sex] => 1
    [subpos] => AO3
    [empStrtDte] => 2012-02-19
)