Kernel/HAL and other internal system files are modified quite frequently. These minor updates don't modify internal structures and that's why avast works correctly on these OSes. However, the problem arises with new leaked Windows OSes/Customer Preview/Beta/RCx versions, because we'd have to change our code to support these OSes (you have to manually download this OS for both platforms and compare the changes). It's worth to do this homework only for "major" releases (like CP/RC/RTM).