Hi DavidR, thanks for your reply.
IF a program has not been detected as containing malware for a certain amount of time (one year, three months, one month, whatever) AND did not undergo a recent update or change, THEN avast might consider a new detection in this program as caused by its own update. Recent effect implies recent cause. Programs that are seldom updated do exist.
Of course the program might be recently infected, but this would count as a change. Does avast not remember some hash digit of executables?
Regards,
Theo Peterbroers