Well Well Well!!!
Again the old story of detection rate!
See my friend, any kind of malware has a behavior which is detected and related to that specific malware as its signature. Since then, the malware is being detected by comparing the suspicious file with that signature.
While there are enormous number of badware and malware, it is very probable that a virus or malware is mistaken with another one, or even with a functional and clean file. Here is the point of difference between the Antiviruses!
It is true that the Antivirus should detect the threats, but it's an important point that the Antivirus detects it accurately. If by any reason a wrong detection occurs, there is a risk that the remediation goes a wrong way and wipes the infection and the footprint inaccurately, and in worst case, the Antivirus captures a systematic file as a bad one and cleans it by delition!!
Hence, a reputable AV such as Symantec never risks substituting the detection rate wisdom with the accuracy. Note that nowadays there are software that generate virus’ or Trojan horses using an old badware. Therefore there are unlimited viruses with minor differences but variable functionality.
I advice to locate the virus and submit it to Symantec technicians and acquire a perfect definition with a perfect remediation, instead of trusting a freeware with a risky behavior!