It's not exactly the octane that makes the difference. Race fuel (fuels that also happen to have higher-than-pump-gas octane) tends to have a slightly lower specific gravity than pump gas. Therefore, if you spray a given volume -- say 1 CC -- of race gas into the engine you've sprayed slightly less weight of fuel than if you'd have sprayed 1 cc of pump gas. Since A/F ratios are a measure of air to fuel by weight, the AFR is slightly leaner because theres less fuel.
Wow, I'm really happy I saw this thread tonight before I take my car to the track tomorow. My car, on 94 octane was runnning between 12.0-12.5:1 at low boost. I then put in C14 and was seeing AFs of 14:1. Is this normal and is it safe to make a couple passes at the track tomorow on those boost levels with that gas?
In the Mk4 Supra's case part of it has to do with whether or not your still running the stock computer. The stock computer automatically retards the timing with lower octane at high boost pressures. Advancing the timing with race gas should have a tendency to lean the car out some. Sunoco 100 octane I know is also oxygenated and this may have something to do with leaning out some also but don't know for sure.