This was a high octane table. The low octane table will have lower timing throughout the table. As you drive, your knock sensor(s) will detect knock (before any audible knock) and will retard timing based on learned "cells". So your normal timing for these cells will be in between the high octance and low octane table. If certain conditions are present, the PCM will dump you to the low octane table if it assumes the you have a bad tank of gas - gas milage will suffer some. After filling up with a predetermiend amount of fuel, the learned timing values will reset and the timing will learn again. Ever feel that the vehicle is more peppy after a fill up? One reason is that the timing has been reset and the engine will produce a little more power.
GM has to design these tables to accomodate different elevations, temperatures (there are separate IAT and ECT timing adder tables), fuel quality (oxygenated or normal), etc.
You will get the best fuel economy when the timing is matched to the fuel quality. A good custom tuner can increase the target A/F ratio from 14.7 to about 14.9-15.0, add timing to the low octane and high octane timing tables (part throttle and WOT), and few other tricks to get better gas mileage while still maintaining acceptable emissions.
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