Because in one part of the manga, they had to enter the book of eibon and one of the chapters was named lust. Once you enter the chapter, you switch genders but depending on how big your lustful urges are... The longer you stay the opposite gender. Strangely, most of the girls stayed boys the longest.
1.) Tsubaki
2.) Liz
3.) Maka
4.) Blair
5.) Patty
They guys were the shortest...