The cause of the anomaly between the Boot Time AVAST scan report and an ordinary AVAST scan is probably simply because, by default, all AVAST’s AV scan setting options have PUP detection turned off. Unless you enable it AVAST doesn’t scan for PUPs.

However the Boot Time option scans for everything and by default PUP detection is turned on. It can be turned off in the Boot Time scan Settings.

To get rid of Open Candy I’d do a manual scan with free Malwarebytes or Spybot. When I last had Open Candy sneak onto my computer both picked it up during my regular weekly scans.

How it got on to my system when I have AVAST set to include PUPs and everything I download manually is scanned with Malwarebytes as well suggests it was integrated into some otherwise trustworthy installer or update software.