When you put it like that, it seems rather obvious
However, each house doesn't get it's own transformer on the pole. I don't know how many houses are served by that big can on the pole, but I think it's at least a dozen.
This morning I tried to replicate the problem without the car. Using my AC, water heater, and a water kettle (I forgot the dryer!), I was able to pull about 30A and measure a 10V drop above and below the main breaker. This is consistent with my findings from yesterday... 233V with little load, ~10V drop at 30A, ~20V drop at 40A... measured over and over again at various spots from above the main breaker all the way to the 14r-50. I haven't opened up the panel around the meter yet, but I will (nor have I climbed the roof to measure the top of the service drop).
I might just ask my neighbors if I can measure the voltage at their houses. I wouldn't even need to open their panel. Probing an exterior 110v socket would be enough.
Thanks
@MikeJr74, this is all good food for thought.