Is it because $_GET is global or the form submitting to itself or else?

I am trying to understand one simple thing in PHP form handling.I am new to it and I have a sample code:

<form name="frm" method="post" action="">
   Item Name:<input type="text" name="itmName" id="itmName"/><br/><br/>
   <input type="submit" name="sbmit" value="Add Record"/>
</form>

<?php
 if(isset($_GET['m']))
 {
   echo '<script type="text/javascript">alert("'.$_GET['m'].'");</script>';  
 }
 if(isset($_POST['sbmit']))
 {
  header("location:1.php?m=10");
 }
?>

Irrespective of what data I send to the server, my focus is on the if(isset($_GET['m'])) part of the code. Everytime I submit the form, the 'if' is always evaluated to true and as a result the alert box appears.Is it because $_GET is holding the previous value set by header("location:1.php?m=10"); or is it because the form is submitting to itself or else?Googling didn't provide much help. I need better understanding over this.With Thanks