And that is quite normal, this is called "the so-called “vulnerability window”. The vulnerability gap cannot be closed when we have no detection and no more malware samples are available because the malware at hand has been closed or is not responding anymore, we call that “dead” malware. You can check this via VirusWatch migration patterns as you are well aware how to. Then because of the engines used we can come to different results. I personally have found that avast and for example DrWeb’s are complementary v.v. That is why a lot of folks close this vulnerability gap by using some additional non-residential on demand scanning programs like MBAM, SAS, and for instance the in-browser Bitdefender’s QuickScan. Also there is a time-related vulnerability window. For a new malware there is detection and within a certain time frame other av solutions add detection. Then there also is unique persistent malware that can go under the av radar until it is being found up, then it is being left, the malware launch will be migrated and a new unique malware is spread from there (Zeus, SpyEye). There IDS malware rule detection can play an important role in early detection looking for anomalities.
Then remember as there is no file detection apparently with avast, that does not mean we are not being protected either by the Webshield or the Networkshield. So have all your avast shields up and running, , you avast knights, and watch your clicks,