It's probably the main more common viruses stored on the disk. To keep a defs file that would track every virus would mean multiple diskettes being swapped in and out. Since we are seeing many of the same viruses/worms over and over again (ie., MyDoom, SoBig, Sasser, etc.) it saves on some time by going after those ones since that's what most people would be affected with today. It'd be rare to see someone infected with .. say... NATAS (a particularly nasty MBR virus I remember being infected with once due to a shrink wrapped diagnostic diskette from 3Com being originally infected)
