Their own site. First of all, political scientists don't use "left center" and "right center." Because that puts a loaded word first and effectively erases anything after it. Leaving only left and right.

Second their left/right scheme puts objective facts based reporting in the "left center" category. Organizations like NYT, BBC, WAPO, etc. and in "right center" they're sneaking in libertarian organizations that want to destroy the federal government and empower corporations in its place. Which gives them credibility they don't deserve while conveniently painting objective reporting as left wing. They even rated a GOP campaign organization as "right center". It's literal political advertisement.

Third, their credibility ratings are highly subjective and rely on cherry picked evidence. For example NYT and Guardian have similar numbers of failed fact checks. But NYT gets highly credible while Guardian gets mixed. The same level as Breitbart; who has double the number of failed fact checks.

Really the more you actually check, the worse it gets.