It is showing how much energy you have scooped with your energy scoop while doing a realspace route. If you have an energy scoop you will always scoop energy when you realspace. When you energy holds are full it will show you scooping 0 energy.

Because if you have a large power tech level and you are out of energy it will show how much you are scooping each pass on the trade route until it gets full which shows 0 sacooped.

It is NOT changing as it is IS useful.