Do the math, look at the specs... an 11.85Th Batch 6 unit draws 1172W plus 12%, or 1313 watts. Push that to 12.93Th and the spec is 1275W plus 7% or 1364 watts, over the 1320 safe limit.
While it is certainly possible that Bitmain produce the B6 "lower power/hash" S9 to accommodate USA 120V users, my bet its that they have much better yields, and much better working hardware at the lower hashrate. I think the lower power draw is just a side benefit (and not the actual reason). It's quite likely that a Batch6 S9 is a set of blades that don't really make the grade as a Batch7 at 12.93TH. You could look at a Batch6 as a "low grade" Batch7, with a price break to match.
Kinda like the way Intel "speed grades" their various processors.
Actually, I tend to agree with you. Suspect the math is just a fluke.
Regarding using "Hospital grade" connectors - your key was in using 10 gauge wire - the standard for 30 amp circuits. Of course if you use 30 amp wiring with 30 amp connectors and 30 amp breakers you can pull more current than if any of those are 20 amp components...