And lets not forget that code (at least in commercial situations) is that breakers should not be loaded to more than 80% of their capacity on a continuous basis. So a 20 amp, 110V circuit (typically using 12 gauge wire) should not have more than 1760 watts of continuous load. Basically one S4/s7/s9 per 20 amp circuit. That limit is 1320 watts if your running with a 15 amp breaker (and typically 14 gauge wire). E.g. Your really pushing your circuits if your running an S9 at over 11.83THs. Maybe that is why they came out with that box? So it would, barely, fit on a typical US home circuit?
Do you know why are did you read it some were ?
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.