It isn't just that avast believes it is compatible with windows 8 but an avast team was at Redmond for some time going through the windows compatibility certification, which they passed.
Like I posted above. There is a world of difference between the results you get in a lab and what you get in the real world. Windows 8 is a moving product at this point. Yes there is a W8-RTM but you will eventually see a W8-SP1 that will incorporate the changes to W8 since W8-RTM.
Being certified is a nice marketing tool. However, it is in the real world that foot strikes the pavement. There are numerous apps out there that were not certified by Microsoft as W8 compatible but still work perfectly fine on W8.
Certification by Microsoft is no different than Virus test files. They have little baring on the real world.
I designed enough systems to know that you can work with IT all you want which no doubt helps but in the end it is where the user sits that counts and those results are often very differnet than what you get in the design rooms