They all develop own methods and tools which detect software that could be a virus. They don't tell other people how they detect these (makes it easier for virus makes to avoid and the competition to steal your work and tools). So each company is only going to be able to detect a certain subset of the know redirectors, malware, spyware, viruses, rootkits, pop-up, etc, etc. No one single software can detect them all. I've had malwarebytes tell me a system was clean when I could look at the temp folders, registry keys, etc and know I had some kind of bug.