LogFAQs > #975905105

Akagi played actual Mahjong with a slight rule twist. Kaiji played what is essentially Yugioh Mahjong, where they skip all the build up bullshit and both of them work with Riichis, or near completed hands (usually you'd discard and set up til you get a good build every single round).

The author just really likes Mahjong in general. We're lucky he only stuck to a single arc, but the twist at the end and Kaiji's cookin' is probably one of the best in the series