 If each team wins in proportion to its quality, A's probability of winning would be 1.25 / (1.25 + 0.8), which equals 502 / (502 + 402), the Pythagorean formula.The same relationship is true for any number of runs scored and allowed, as can be seen by writing the "quality" probability as [50/40] / [ 50/40 + 40/50], and clearing fractions.The assumption that one measure of the quality of a team is given by the ratio of its runs scored to allowed is both natural and plausible; this is the formula by which individual victories (games) are determined.