Sometimes i cleaned registry of a program and then later I see another key there. Some installs are real sticky and hard to untangle. Cases where more entries turn up unexpectantly are a bit of a mystery but mostly sorted like where you found the suspect key, deleted, and ran the avast again. Thats what I have found anyway.
But its the AV leftover remnants that cause the problems thats for sure. With most programs a few loose keys and untidy bits are nothing of a problem, but with AV it seems always that there can be problems. Maybe yes, maybe no, but always potential there. And twice as bad with Symantec.,