I know these codes are only used for billing purposes, but I always wondered why they are numbered/ordered the way they are. Can anyone shed some light on why the decimal digits skip so many places and why the PD’s are ordered this way? For example why aren’t the groups (Cluster A, B and C) grouped together–Avoidant PD (Cluster C) is between Narcissistic (Cluster B) and Borderline (Cluster B). That doesn’t make any sense to me. The same thing with Dependent (Cluster C) being between Antisocial and Histrionic (both Cluster B). Are these numbers just assigned randomly or is there a reason? Maybe a psychiatrist or medical billing expert here can answer this.
I know it doesn’t matter but I’ve always been curious about this.

301.20 Schizoid
Personality Disorder

301.22 Schizotypal
Personality Disorder

301.4 Obsessive-Compulsive
Personality Disorder

301.50 Histrionic
Personality Disorder

301.6 Dependent
Personality Disorder

301.7 Antisocial
Personality Disorder

301.81 Narcissistic
Personality Disorder

301.82 Avoidant
Personality Disorder

301.83 Borderline
Personality Disorder

301.9 Personality
Disorder NOS