Lambda 1.0 and perfect combustion

In this tuning demonstration, peak torque @ 2,000rpm is produced with a lambda of 0.93. If lambda 1.0 represents perfect combustion for a given fuel, why are we seeing power gains as we add fuel up until 0.93? Is this a byproduct of cylinder cooling? Thanks!

Lambda 1 produces the ideal burn at specific conditions that are not always met while engine is going through its cycles resulting in not all the oxygen molecules being used. Adding slightly more fuel helps to use those residual oxygen molecules thus extracting more energy and creating bigger pressure inside of combustion chamber which in its turn results in bigger torque and power

As georg said, a perfect chemically balanced burn is difficult to achieve, so a little more fuel is commonly used for maximising cylinder pressure - torque/power from the engine. There is also a 'lean best' condition where the mixture is leaned out and it is to extract the maximum cylinder pressure - torque/power for a given amount of fuel. To put it simply, one gives you more speed, the other gives you more economy - getting the transition right is where it can get interesting.

However, there are a few other factors, such as the evaporative cooling for charge density you mentioned, richer mixtures often being slightly slower burning so more timing is possible to better place the peak cylinder pressure, etc.

I'ts all down to combustion efficiency, intake and exhaust port design and combustion chamber design. GDI engines can just about hit MBT at lambda 1.0 but PFI engines need a little extra to make sure there is a rich enough mixture around the spark plug to initiate the burn. You will often find the AFR at which MBT is achieved varies across the load and speed range due to the speed of the incoming charge and gas exchange dynamics of the intake and exhaust system.

I appreciate the responses! It's been helpful.