Hold speeds
Yet HazelNuts39 has tackled an issue that might open the way to explaining this mystery.....
i have looked again at the all the engines that i mentioned above in my previous posts....
the only difference that i found between all these engines and the V2500, is that they all have bigger fan diameters / higher blade tip speeds and higher bypass ratios of a range 5.6 to 6.2 while V2500 has only bypass ratio of 4.9 which indeed affects the specific fuel consumption for each engine type (V2500 has specific fuel consumption of 0.560, compared to 0.591 of the CFM56), and might require the aircraft to be flown at speeds different from G-dot for each engine type, in order to maintain the speed for best fuel consumption for each specific engine.
Now if we consider: 1- that the G-dot speed is related to the wing and airframe rather than the engine itself (close to the best lift/drag speed).
2- that the A320 initial design and characteristic speed calculations were based on aircraft fitted with CFM56 engines rather than the V2500 which was not introduced until a later time.
then it would only make sense that this difference would impact all aspects of performance, and would show on the Perf. tables.