Walker has never even scored runs at the highest rate on his own team. He just never gets promoted early and has stayed healthy, while playing with other good hitters. If you looked league wide, his run scoring rate is nothing special at all. The good players had less games due to promotions. In rookie ball, he scored one less run than Pimentel. In low-A, he was 4th on his own team in runs scored per game, and it wasn't even close. Buxton and Williams scored 50% more often per game, and Polanco scored slightly more often as well. In high-A, he scored fewer runs than Harrison despite playing in more games, and scored much less per game than Polanco. In AA, Walker has scored 5 less runs than Sano despite 9 more games, Buxton had a much higher rate per game, and Kepler's per game rate is basically even.