At a quick glance this is what it looks like you have happening:
Person clicks a radio button then submits it to the page you displayed here.
The page you displayed here lets the person verify that was the one they wanted to vote for and re-submits it to the same page.
The page then puts it in the database.
If this is what happens, then the problem is you aren't re-passing the variable to the next instance of the page after they verify they picked the right choice. You can either do that with a hidden submit or a session variable:
$_SESSION[selected_radio] = $_POST[$vote];
mysql_query("INSERT INTO voting (username, cluster,vote) VALUES('$netid', '70s','$_SESSION[selected_radio]') ") or die(mysql_error());
Michael