It really depends on what grade of gas you use. Lower octane rating'd fuel burns faster than gasoline of a higher octane rating. If you gain fuel economy from retarding the timing, then you're probably using a lower grade of fuel than was originally intended for the engine, and once you've retarded the timing, the engine is able to better use the fuel burn to move the car. With a higher octane rating, you can get a longer useful fuel burn, so you'll need a bit of advance compared to a lower grade fuel.

At the same time, you can advance the timing to get a better throttle response, but throttle response doesn't equate to more power or better economy, just better throttle response. However, if your timing is advanced too far to make the best use of the fuel while burning, it'll get the piston moving faster, but not really have any follow-through. That make any sense?