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MisunderestimatedChesbro, Marquard (and to a lesser extent Willis) have relatively weak career numbers for HOF pitchers.
Both "Rube" and Happy Jack" got elected in the pre-SABR era so legends and memories trumped cold hard stats (which weren't even available until about 1969-70)
Chesbro had a very short career for a HOFer (11 yrs) which included 2 great seasons and an exceptional lifetime winning percentage. In one of those seasons (1904) he set the so-called "modern" single season win record. He's sort of the pitching equivalent of Roger Maris.
Marquard's case is weaker. He started with the Giants who paid big money for him as their new Matty and after a few season set a record (or it was recognized as such) for consecutive wins that was a very big deal for many years. After he won 20 games for them for 3 years in a row he drifted towards mediocrity but stayed in the majors long enough to eek out 200 wins.
Willis oddly enough got elected after stats took over and was rescued for oblivion even though his numbers didn't really separate him from a number of pitchers from his era (1890's-1900's) who never got in. His election while perhaps more justified in hindsight is more perplexing to me than those of Chesbro or even Marquard.