Missed this post - sorry.
The armor unit ratings are not so much about the type of tank as they are about the organization, tactics and leadership.
For example. many French tanks were superior to German tanks for the invasion of France - and many of the French tanks were heavier that German tanks and had a heaver armament. But, the Germans organized and used their armor much more effectively.
So, a "medium armor" unit really reflects a more effective use of armor and combined arms than is does tank type. "Light armor" represents a less developed organization and less developed tactics for the use of armor formations.
Early in the war, the majority of German armor formations were made of of tanks even light than PzKw III's - there were many PZKW II's and 38-t's (a Czech tank aI believe) that made up German panzer divisions. Taken individually, they were inferior to many of the French, British and Russian tanks they encountered. However, taken as a formation, they almost always defeated French, British and Russian formations early in the war.
This is why German armor formations (called "Medium armor" for simplicity) are given a rating of 6-5 right from the start of the game and most other nationalities, yet to develop and implement better armor organization and tactics, are given a rating of 5-4 for their armor.