Some background on AFR tuning. Stoich mixture is 14.7:1 AFR; which is 14.7 parts air to 1 part fuel by mass. This results in cleanest burn with most of fuel and air combusted. Highest combustion temperatures and lowest combination of emissions gasses:
However, this really only works at partial-throttle steady-speed cruising with plenty of time for combustion. When you open up throttle and want more power for passing and acceleration, we want slightly richer 12:1 to 13.5:1 ratio. Why?
Because of space and volume and incomplete mixing of fuel and air. Due to incomplete atomisation and dispersion of petrol in combustion chamber, there are areas with leaner and and some areas richer than 14.7:1 ratios. This results in some areas with extra oxygen molecules with no fuel to combust with. So a little extra fuel is needed to supplement lean spots to ensure that all oxygen has matching fuel for maximum combustion pressure (BMEP). However, too much fuel is no good either as extra petrol cools and slows down combustion, resulting in incomplete combustion and lower cylinder pressure and power.
On carbed engines this max-power enriched mixture is around 12.5-13.0:1 AFR.
Fuel-injected engines gets max-power around 13.0-13.5:1 AFR due to better fuel-atomisation.
Newer direct-injected engines have even better mixing and can maintain 14.0-14.7:1 AFR under WOT for maximum power.