You need to look at your system as a whole, and examine the ports you have fitted - it would seem that some of them are using drivers which report the name incorrectly - we can't duplicate that here as we do not have access to your system.
If you can't do anything about it, then one way to "hide" the problem would be to use a regext to get rid of any "rubbish" at the end.
string[] ports = SerialPort.GetPortNames();
foreach (string port in ports)
{
string cleaned = Regex.Match(port, @"COM\d+").Value;
cboPorts.Items.Add(cleaned);
}