That actually appears correct using techpowerup's performance summary numbers. 980 -> 980 ti, 780 ti -> 980 and 390x->fury x all were smaller jumps. You can argue overclocking and such but it's not unfounded to say.
Yeah, I'm sure there are reviews out there to support that. In the real world we all know the gap between 980Ti and 980 is bigger than 1080 to 980Ti.
But it's not important, silly "article".
