Norton 360 Premier Anti-virus Scan missed 10 viruses

What is going on with the Norton 360 Premier Antivirus? I've been using it now for at least 4 years and it never found a virus (or so it reports) and only an occasional tracking cookie. My friend has been "bugging" me to try another anti-virus and so I gave in today. I first ran a full system scan using my Norton 360 Premier antivirus and it found nothing. I rebooted and ran the other antivirus. It found ten, yes - 10, viruses, including phishing and key stroke trackers in both programs and my registry. Doesn't Norton scan for all viruses in all areas or does it take "shortcuts" and leave many areas uncheck and vulnerable? And if it does leave vulnerabilities why should I stay with Norton and Symantec?