If you are looking strictly at the scoreline of their 2015 Roland Garros match, Djokovic kicked Nadal's ass. If you look at the level of play, particularly in the first and second sets, it was not a completely dominant performance by Djokovic. This is completely consistent with my comments about Federer-Nadal matches on clay; I have said many times that the level was very close, but Nadal pulled out key points. There is no inconsistency here. There is just some complexity that flies over your head.
But, if you want to compare scorelines from the matches you brought up, here you go:
Nadal v. Gabashvili, 2009 US Open: 7-6, 7-6, 6-3
Nadal v. Istomin, 2009 US Open: 6-2, 7-6, 7-5
Djokovic v. Nadal, 2015 Roland Garros: 7-5, 6-3, 6-1
What's the difference? The two Nadal victories were clearly much more complicated. In the Gabashvili match, in particular, Nadal genuinely struggled. It's not too often he goes to two tiebreaks in the first round of a Grand Slam.
It's 2020. You have been on the forums for at least 15 years. It's clear that you have less vocabulary at your disposal than a 9-year-old, and you equate any sort of complexity with logical fallacy. As I explained above, there was no logical fallacy in what I said. And of course what you call "bias" is simply anything you don't like the sound of.