Avast Antivirus Finds System Junk that CCleaner left behind?

[rcpiriform]

Avast Antivirus finds 3.08GB that CCleaner Pro left behind.

Attached please find a notification from Avast Antivirus indicating that 3.08GB of junk files are present, this is after I have used CCleaner Pro clean-out tools.

[nukecad]

Different cleaners target different things.
CCleaner itself can be customised to target different things.

Without knowing specifically what the 3.08GB of junk files your Avast found that your CCleaner didn't were then it's impossible to say more.
Can you get Avast to show what they are/were?

Are you using Health Check or Custom Clean?
Health Check uses it's own rules, but It's quite likely that if you use Custom Clean that 3.08GB will be in one of the catagories that you can clean there - if selected for custom cleaning.

Registry Items -
CCleaner includes a Registry Cleaner, you run it seperately because it is only recommended to use if you actually have a problem with the registry.
Cleaning the registry when you don't have a problem can cause problems, and even 'break' your machine.

Apps slowing down your PC -
CCleaner>Options>Startup, again it's seperate because you may not want to change what starts with your computer.
CCleaner>Performance Optimiser, lets you choose to put apps in the background that are not currently being used to 'sleep'. (It's patented by CCleaner).

PS. Are you aware that Avast and CCleaner are part of the same company now, along with Norton, LifeLock, Avira, AVG, and ReputationDefender? It's called 'GEN Digital'.
https://www.gendigital.com/

[rcpiriform]

Thank you for your informative reply. Will dive into it which will keep me busy for a bit and definitely decrease my ignorance.