Don't forget that GTX 480 also came out half a year after Radeon 5870. When 5870 came out it was competing against the GTX 285. And even then, the GTX 480 and Radeon 5870 were basically even trading leadership depending on which games were tested.
Man, looking back at prior generations, GPU progress has slowed down a LOT for both companies.
Prior generations ~6-9 months per generation (counting mid gen as a generation as performance jumps for mid gen were generally similar to modern day new generations)
200 series to 400 series ~1.75 years
400 series to 500 series ~7 months
500 series to 600 series ~1.25 years
600 series to 700 series was ~1 year.
700 series to 900 series was ~1.5 years.
900 series to 1000 series was ~1.66 years
1000 series to 2000 series was ~2.33 years
Looking at that it also becomes quite evident that GTX 480 was significantly delayed. NV had problems getting the chip to where they wanted it be. And even then it was a HOT running chip that consumed a lot of electricity, especially when compared to the 5870.
Also, while NV were making really large chips, ATI/AMD were making relatively speaking much smaller chips. So while NV had the performance crown, AMD had the perf/mm and perf/watt crown up until Maxwell. Radeon 2900 really scarred ATI/AMD WRT building large chips for a long time.
AMD progress is even slower. And I don't expect things to get better from here on out. Then again as old as I am now, 2 years feels like how 1 year used to feel.
Regards,
SB