Stock vBIOS. That's a chart from techpowerup's review and they didn't say anything about a custom or oc bios on the review so I'm assuming the review is on stock bios.
Well... How's EVGA going to know what's the rest of your hardware? the 750W recommendation is a rough estimate.
Probably EVGA calculated something like: 530W (max on GPU) + 120 W for CPU + 100 W (for MB, RAM, etc) = 750 W
You might have a cryo cooler or some crazy hardware that sucks a lot of power and there is no way EVGA is going to know.