Why does the pot call the kettle black then the pot is black not the kettle should it not be the other way around?

The Pot Calling The Kettle Black = to criticize someone else for something that could equally well apply to oneself. (suggests a certain blindness to one's own characteristics)

Example: "Ah, look who finally decided to join us: Mr. Tardy, himself."

Response: "Aren't you the pot calling the kettle black! You're always the last one to show up to everything!" (Takes one to know one!)

Background: In medieval kitchens, "the pot" (for cooking food) and "the kettle" (for boiling water) were made from heavy materials like cast iron to withstand the heat from an open fire. The cast iron would become black with use, collecting oil, food residue, and soot from the flames. The pot sees the blackness on the kettle and points it out, while not noticing that same blackness on itself. -- Phrase originates in a 1620 translation of Cervantes' Don Quixote

Just to clarify: The order of the phrase is actually irrelevant. Whether it is the pot calling the kettle black or vice versa, the effect is the same. The point is that while both are flawed (ie black), one cannot see its own flaws.