Whats with the different Anti Virus programs?

Zinc

Zinc

Adept
I've been using Norton Internet Security for quite a while and been very happy with it as it does its job quietly in the background and I haven't noticed any considerable slow down unlike its previous older versions.

Sometimes I use my pen drive on some other system like a friend's or in a cafe to take printouts and all and more often than not it happens that their anti virus shows up threats on my pen drive and cleans it up.

Later when I plug it in my system then my NIS shows up threats and cleans them up.

This happens over multiple systems and not any specific ones, the only common thing being my system and my NIS.

I scanned using Malwarebytes' Anti-malware and it showed nothing.

I can also use some online scanners but they don't seem too reliable.

I can't use a different program for at least 6 months.

So what's the reason for this?

TIA
 
@b00gieMan: As I said, the only thing common is my system and it happens with different pen drives also. Files usually include tickets, text and Word documents, sometimes saved game files from PS3, etc.

@ayanavish: I think its a system issue since it happens with different pen drives but I'll try that.
 
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