Sure, he either slowed down his driving or he was not being fully honest. Most computer chips will only modify things like timing and fuel flow so much. And most of them really only work at wide open throttle settings. Here is the problems. The computer looks at Mass Air Flow sensor voltage to determine the amount of air going into the engine. With that information, the computer calculated fuel flow. When the burn occurs, the spent exhaust gasses leave the combustion chamber and flow past the oxygen sensors. Those sensors send information to the computer as to how much or how little unburnt fuel is in the exhaust system. The computer uses that to modify its fuel calculations. Kind of like doing a math problem backwards to check the answer. We call that closed loop.
At part throttle operation, the computer is doing that comparison constantly. Any modifications to the fuel look up tables will be compensated for by the O2 sensor outputs. Where that changes is when you are operating at wide open throttle (max power) When the computer sees greater the 90% throttle, the o2 sensors are no longer part of the calculation and any programming will have an effect.
When chip designers are changing the parameters of the computer operation, their biggest concern (and target market) is the performance crowd. Hense, what they are able to do is generate greater horsepower, without having a detremental effect on fuel economy.
With spark cuves, you hae the same problems, especially with engines (like yours) that hae knock sensors. Again the timing is advanced until the spark knock is heard by the senosr and then it is reduced slightly. On a modern engine, the timing correction can happen from one firing event to the next. And at 3000 rpm that means within .04 seconds (if my math is correct). Again there is not much that the programmer of a chip can do to change that. Keep in mind that this is all occuring during part throttle operation.
There is just only so much that can be done with a fuel that has 67% of the energy of 91 octane gasoline.
And by the way, that is about what you are getting (61%)