the webrep ssytem isn’t ALWAYS reliable, and sometimes false positives are a thing.
Basically, if a site is listed as ‘green’, it doesn’t have a reputation of hosting malware. However avast will still block a page if it thinks it’s spotted any malware embedded in it, regardless of the site’s reputation.
sometimes the malware might not even be from the site itself, but from an ad embedded in it or something, or it could simply be a false positive.