The shape of Mantle's and Clemens's careers are somewhat different, with Mantle having a higher pWORL than Clemens's career-best in 2 different seasons. Clemens makes up for Mantle's better peak, however, by putting together an incredible string of 24 consecutive above-average seasons.
As I noted above, Mantle and Clemens are very close to each other in career pWOPA and pWORL. Interestingly, they are even closer in pLosses (2.5). But Mickey Mantle accumulated 33.7 more pWins than Roger Clemens and, consequently, had a higher career winning percentage.
What then brings Clemens back up to Mickey Mantle's level in pWOPA and pWORL is an offsetting difference in the two players' positional average
In his career, players who played the same position(s) as Mickey Mantle (mostly CF) amassed an average Player winning percentage of 0.518. Mickey Mantle spent his entire career in leagues in which pitchers batted. Because pitchers are well below-average offensively, everybody else - including, of course, center fielders - is above average offensively, basically by definition. This serves to raise the positional average against which Mickey Mantle is compared in his career.
In contrast, players who played the same position(s) as Roger Clemens (i.e., starting pitchers) amassed an average Player winning percentage of 0.489. On average, starting pitchers are, of course, well below average offensively. In addition, starting pitchers
have a lower average winning percentage than relief pitchers. Both of these factors serve to lower the positional average against which Roger Clemens is compared in his career.
I set replacement level
one weighted standard deviation
below positional average, with separate standard deviations calculated for pitchers and non-pitchers. The standard deviation for non-pitcher winning percentage during Mantle's career is 0.041, leading to a positional replacement level for Mantle of 0.477. In contrast, the standard deviation for pitchers over Clemens's career was 0.052, for a replacement level for Clemens of 0.437.
Put it all together and we get two very different players with very similar career values.