My fuel economy suddenly got better...for some reason?

There are many factors the can effect your fuel mileage. The weight you have in the car. The type of road you are driving (hills), the weather conditions (Head winds or tail winds) the speed you are traveling, and the condition of the engine, the amount of traffic (spent more or less time in a line of cars). There are a bunch of variables that will affect the mileage. Adding a system treatment may have helped some as it allowed the system to run more efficiently as time went on. But I think the main issue is the conditions of the road and the conditions of the weather.