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Between 1900 and 1916, the New York Giants went 1,456-1,080, good for a .574 winning percentage during Mathewson's career. Mathewson went 372-188 with the Giants, giving him a .664 winning percentage. In games where Mathewson did not factor in the decision, the Giants were 1,084-892, a .549 winning percentage.

Between 1907 and 1927, the Washington Senators went 1,559-1,609, giving them a .492 winning percentage during Johnson's career. Johnson went 417-279 with the Senators, giving him a .599 winning percentage. In games where Johnson did not factor in the decision, the Senators were 1,142-1,330, a .462 winning percentage

What you are really doing is comparing Johnson and Mathewson to the rest of their teams pitchers. The record of the starting pitcher has nothing to do with the quality of the other pitchers on the staff, it has to do with the run support received. If Johnson received significantly less run support then Mathewson that would matter. If Johnson pitched on a staff that was considerably weaker then the staff that Mathewson pitched with that would not matter at all in comparing them.