If you did then you would understand that there's not enough for all the Carbons and Oxygen molecules to pair up. That means there's excess Carbon to find its way onto the plug.
Lambda is different for different fuels.
Using Lambda to Determine Air/Fuel Ratio
The result of the calculation is the ‘Lambda’ (l) a dimensionless term that relates nicely to the stoichiometric value of air to fuel. At the stoichiometric point, Lambda = 1.000. A Lambda value of 1.050 is 5.0% lean, and a Lambda value of 0.950 is 5.0% rich. Once Lambda is calculated, A/F ratio can be easily determined by simply multiplying Lambda times the stoichiometric
A/F ratio for the fuel used – e.g. 14.71 for gasoline, 15.87 for LPG, and 17.45 for CNG.
So you still think an O2 sensor tells you the AFR in different parts of the cylinder? Maybe if you hung out at Innovates Forum or spent enough time there Bruce clearly explained how AFR isn't AFR because it reads the combustion after the fact. Nor does it tell you what the HC ppm is and the Carbon gas balance.
I can change a plug read to make it look Fat Azz with out changing the AFR one bit. So if the AFR I give the engine didn't change then how can it look BLACK...........
Let me know when you figure it out.