The idea behind it is probably that....
GNW Ninja are stronger because that's who they are at the last part of the story....
They are at their strongest point in the end basically.

While their normal self ninja(like picture below the GNW) is who they were throughout the story.
Yes they could have been strong but they weren't their strongest until the GNW happened. That's where they had to try their hardest and believed they could do anything and win.

So yes....GNW being the same in power would make no sense lol!