If the gasoline was a mixture of heptane and octane, this would be exactly the case. I don't know if gasoline ever was a mixture of heptane and octane, but it is not now. Otherwise it would be impossible to achieve greater than 100 octane gasoline.

There is still something like a base material with additives used to increase knock resistance, and the proportion of the additives will likely be close to linear with the octane number assigned. It is unlikely to be exactly proportional, but since you're talking about a small difference, it will be close enough.