This could indicate that the gasoline has ethanol in it. Ethanol has a very high octane rating. But ethanol is also very volatile so it evaporates quickly and leaves the lower octane components of the gasoline behind.
Other high octane blending components, such as FCC gasoline, alkylate, or reformate, are used in higher percentages in non-ethanol gasoline. Those components should not evaporate significantly faster or slower than the rest of the bulk gasoline.
Octane could increase if you are using a winter gasoline and it gets hot outside. Butane is added to winter gasoline for higher volatility for faster starting. Butane has low octane. If it evaporates out of the gasoline faster than the rest of the components, you will have a higher octane gasoline. (But less of it.)