I'm not sure whether this had been brought up before but nonetheless it seems to me that corporate version of symantec antivirus are far more superior in terms of virus detection. Recently I performed a scan on a file which was cleared by my NIS 2010 but when I submit it to a web based antivirus scanner that's using symantec corporate AV(might be EndPoint Protection product) for analysis, it flagged the file for virus.
Note: Both of the scanner are using up to date virus definition files.