The scale was created in the 1970s after testing was done by applying different ingredients to a rabbit's inner ear. Ingredients that caused clogged pores to form were deemed comedogenic with a rating of 0-5 (with 5 being likely to clog pores).
This is wrong on many levels, and it's clear how the very nature of this testing is totally inaccurate.
A rabbit's ear is simply not the same as human skin.