Not the dev, but they probably did it this way for simplicity, instead of coding 18 different types, they made 6 groups of three so its easier to balance. Don't get hung up on the original games types chart, the only onde you need to learn, and that takes 2 minutes, is WHITE > BLACK > YELLOW > WHITE.
Just as a fun fact, here's all the original types inside every Pokémon Overlord type:
White: Normal, Flying and Fairy
Black: Dark, Psychic and Ghost
Yellow: Electric, Fighting and Steel
Red: Fire, Ground and Rock
Blue: Water, Ice and Dragon
Green: Grass, Bug and Poison