Could it be they had a bad copy of the 7D for testing? I've always remembered my old 7D focusing a bit better than my 60D.
I guess you didn't do any (semi-)scientific tests? I don't have a 7d, but I've read many times that the af on the 7d is the least precise, and the lensrentals test confirms it - though by a margin you may not notice in day-to-day shooting because unfortunately the 60d is rather unprecise, too, and of course the lens also matters.
I compared to my 5D III and I didn't see the advantage of 6D has to offer in term of -3EV AF center point. Maybe it wasn't dark enough? I shot with my 24-70 f2.8 II lens at f2.8. The outer points were bit slower though.
The good news for 6d buyers is that obviously you didn't see any disadvantage, too - and yes, I guess it wasn't dark enough because -3lv is rather dark, though not different enough from -2lv to tell a difference w/o a lightmeter (or you can look inside your picture files, the measured light value is in there). From kr http://www.kenrockwell.com/tech/ev.htm
LV 3 Brightly lit night street scenes
LV 2 Typical night street scenes
LV 1 Dark scenes outdoors at night
LV 0 LV Zero is defined as the light level that requires a 1 second exposure at f/1 with ISO/ASA100 speed film.
LV-5 Scene lit by the full moon
LV-15 Scene lit only by starlight.