I'm not sure why that is, other than that I think the 'from' field is usually the field you would bind the sender's email address to. The email would need to have a 'sender'. So in mine, If the email was being sent to me as the site owner, then I would bind the form's email address field to to the 'from' field via the lightening bolt:
$MailFrom = "".((isset($_POST["Email_Address"]))?$_POST["Email_Address"]:"") ."";
I would add the Name binding to the email body.