Newer games are increasingly being developed on very well threaded game engines. And with the PS4/Xbox One being developed on 8core Jaguar chips we are more likely to see games that use more than 2-3 cores in games. This would lead to chips like the i7 series and 83xx series getting more FPS than lower core chips. Also remember that even if a game for instance only uses 6 cores the 83xx would still be faster because the remaining cores will help the OS do background work. (Small but sometimes noticeable differences).
Besides that the Vishera FX chips are designed in such a way that certain games that use a lot of FPU math (decimal math) may be happier with an 8 core chip than the 6 core. (Every two cores has 1 FPU). So even though the FX chip is 8 core it really is 8 Core INT, 4 core FPU. Again just depends on the games you play and software you use.