Why this isn't actually an "AVG" check
Antivirus vendors like AVG and Avast scan files and local system activity on a device you've installed their software on - they don't offer a public API for looking up arbitrary third-party URLs the way this tool's name implies. Rather than fake that, this honestly uses Google's Safe Browsing threat database instead, which is a real, continuously-updated source covering malware and phishing sites across the web.
What Google Safe Browsing actually covers
Safe Browsing is the same real-time threat intelligence system that powers the built-in warnings in Chrome, Firefox, and Safari when you try to visit a known dangerous site - it tracks confirmed malware distribution sites, phishing pages, and sites pushing unwanted software, updated continuously from Google's own web crawling and reports from across the security industry.
Why a "not flagged" result isn't a complete safety guarantee
A clean result means the URL isn't currently listed in Safe Browsing's database - it doesn't mean the site has been comprehensively audited for every possible risk. A newly created malicious site, for instance, may not be flagged yet simply because it hasn't been discovered and confirmed by any threat database.
Why different security tools can disagree
Different security vendors maintain their own independently-built threat databases with different coverage and update timing - a URL flagged by one vendor's database may not yet appear in another's, and vice versa, simply because each operates its own detection and confirmation pipeline on its own schedule.