In actuallity, it does NOT pull directly off the house, that would require additional wires in the car, basicly wiring from the on board charger and the inverter and going directly to the heat/cool instead of the OBC, inverter, battery then heat/cool (current set up).
So the heat/cool pulls directly from the battery and the incoming power from the house recharges the battery as its being depleted. If the power draw is too much to keep up the battery remains slightly deficient and hence the lower SoC. This is also proven with the Leaf Spy app tapping into the # of Level 1/Level 2 charges, if you preheat after the car has been full (either 80% or 100%), the car counts it as another charging event (pretty sure unless I was seeing something else on that #).
In addition if it was running directly off the home power, it would just use less when the car had reach the target temperature, but if you had set your car at 80% charge and you preheat/cool and you reach your temperature and use less power, the car will keep charging the battery (at full power) until you stop the preheat/cool, which could leave your battery at a greater than 80% capacity.
This is actually quite contrary to Nissan's statements of "not top off the battery" but "preheat/cool to get greater range" You can't have both.