People write it, and those people are human.
They argue among themselves. http://www.sagepub.com/upm-data/58622_Chapter_7_Pomerantz_%28Clinical%29_I_Proof_%28Low_Res%29_4.pdf
The actual Bible is less contentious than the DSM. Everyone reads it, and no one, NO ONE, is happy with it. Everyone ignores sections or bolsters others. The reading is subjective to every clinician.