no security program have 100% detection…
and something like 100 000 new malware samples is found every day, so to detect it all is mission impossible

and if you check any removal forum like Malwarebytes forum / Bleepingcomputer / Geeks to go …
you will see from the removal help sections that this is not only related to avast … no AV detect everything

also antivirus programs have a general cleaning procedure and often leave leftover files … this can be spotted using the tools above