User blog comment:Gilgamesh79/Question:Multiple Evil Piece consumed character/@comment-31137559-20180730153711

How many evil pieces a person consumes depends on their current power level/potential. Their power isn't the result of how many evil pieces they consumed, but rather the other way around.

We can accurately say that Macgregor Mathers was an extraordinarily strong magician while he was human for someone as powerful Sirzechs to have required two bishop pieces to reincarnate him into a devil. While the bishop piece did enhance his magical abilities, I don't think it boosted his magical abilities any more than someone who only consumed 1 bishop. But Mathers was just originally an incredibly powerful magician. The 2 bishop pieces didn't make him that way. It was a result of him already being very powerful as a human.

While Okita Souji consumed two knight pieces only because of the Youkai that had taken residence in his body after performing various rituals to save his life. While his speed(and also Kiba's) can be attributed to his fighting style and swordsmanship which largely relies on speed.

It's just that Mathers and Souji were already pretty powerful or had something powerful within them that caused them to consume more than one piece.