Board Thread:Questions and Answers/@comment-24578352-20140222115942/@comment-1821586-20150714151937

Kasumi's weakness is that while only having one suit greatly increases the speed of completing the own hand, also the other three players have it easier with only two suits. In Saki's case this means that she can complete kans faster. The defense is also easier as stated by Kyouko in the manga.

There are 4*9 = 36 tiles of each suit and 4*7 = 28 honor tiles for a total of 136 tiles (there are no seasons or flowers in riichi mahjong).

Each player starts with 13 or 14 tiles and draws 17 or 18 times which means that at least 5 tiles of Kasumi's suit (more if she has or draws honor tiles) must be drawn by another player (probably near the end) or be hidden in the dead wall. If the tiles were in the dead wall then this would hinder Saki, but this isn't the case which means that Saki gets useful replacement tiles after calling a kan.

This was still not enough to compete against Kasumi's advantage, so she had to use Toyone. She collected three of Toyone's winning tiles, and the riichi and pursuit riichi forced her to draw the fourth tile, completing a kan.