Its somewhat of an unsolved problem, there's all sorts of these kind of sites that are legitimate but look really suspicious, like they could be phishing sites. Examples:
https://www.annualcreditreport.com/
https://www.optoutprescreen.com/
You can tell these two are legit because they are listed on official government sites:
https://www.ftc.gov/faq/consumer-protection/get-my-free-cred...
https://www.consumer.ftc.gov/articles/0262-stopping-unsolici...
However, I can see how some people wouldn't think to go through .gov sites and there's really nothing stopping me from registering a phishing site with a similar domain and hosting it in Russia or something to confuse people into using mine rather than the official one.
I think freecreditscore.com has been confused with annualcreditreport.com in the past. Freecreditscore.com is owned by Experian now and it actually does offer a real free no strings attached credit score NOW, but in the past it used to be one of those sites that tricked you into signing up for a subscription.
Then, of course, there was the Equifax data breach site that totally looked like a phishing site.