Kaspersky found some viruses and other whatnot which AVG8 couldn't find.
As very general users I'm not that sure we can really know something like this. I mean, AVG 'finds' viruses on my machine that I'm sure other anti-virus programs will miss... but they're not necessarily viruses, they could just as well be (and actually have been) false positives. I'm not really sure how we're supposed to assess anti-virus software with any degree of certainty -- they seem to be mostly a matter of faith :/