I believe one place the gains may come from is the small amounts of water vapor that are getting into the chamber through our systems.

By the vapor entering the combustion chamber it turns to steam when the combustion happens. This raises the pressure and converts some of the energy normally lost as heat to mechanical energy used to drive the piston back down. Yes it slows the burn, but we are also speeding it up with the hydrogen. I bet the 2 balance one another out.

This is only a hunch so please don't hate me if I am wrong but it does make sense. Especially if there is not an hho dryer installed in the car after the last bubbler before the engine.

Another place we MAY be getting improvements is from the computer sensing the car is running fine on a lower amount of fuel. The cars cpu is programmed to only send as much fuel as is required to keep the sensors happy and in turn the car running smoothly. That being said if we provide a fuel, any fuel, to the car it will compensate by sending less through the injectors.

Now you are all about to post that I am not correct because of the need for the efie. Well to my understanding the efie is needed mainly because of that pesky oxygen molecule. The o2 sensors see that and think there is air not being burned so it ups the amount of fuel to try to burn off the extra oxygen.