[tripzilch]

The difference is that Microsoft has more incentive to optimise their AV against both false positives, and false negatives. And it can afford to stay invisible if there is no threat. A commercial AV has to make its presence known, and most I've seen do this constantly: If you never get a virus and the program remains silent all the time, people will wonder if they really need a commercial AV. If you never experience trouble and Microsoft Defender stays silent, you have a happy user: Windows works without issues.