While Yama is clearly the most famous of the hell magistrates, legend has it that he was too lenient and hence demoted to the fifth hell. I would speculate that such a demotion was actually for artistic composition purposes, the fifth and sixth hells being the center of the scroll sets and hence placing Yama roughly at the center of the display.
For other scrolls that clearly share a design history with this one, see the Donnelly catalog (p. 85) and the Vidor catalog (p. 76).