Your reasoning is rational. It just came across as if you are tuning to AFR instead of tuning to torque and power. Differences in load, fuel distribution, and fuel itself are some of the things that can alter AFR that your engine appears to run best at.

One reason your engine may need to be richer is to provide enough fuel to take care of the leanest cylinder. If you know which cylinders are richer or leaner, then you can try to improve the distribution.

Another is that AFR will also vary with fuel composition and density. Race gas is held very close to whatever the manufacturer publishes. Pump fuel will vary from year to year, region to region and winter to summer.

(The AFR gage is an interpretation of a WBO2 which itself is an interpreted measurement of oxygen. There are conditions that will result in misreadings. Another reason to check the spark plugs.)