An fe'r who is finally posting pictures of observations. Well done.
First my question; do you have a picture from the same distance taken from a higher elevation? Or a picture from much closer?
Where are the bushes and steps between the house and the water? There's another small structure lower on a beach just off the side of your picture (I can make out another house further back under the trees) was that lower structure also visible, but just not framed in the shot? GE shows the parking lot (and I assume the first floor) to be 9 feet.
Also, you claim the drop is 12.9 feet, I see no problem with this, and according to the ENaG chart that's correct. However, that's based off a starting point elevation of zero. If you were 2 feet above the water, that puts the drop starting point out further (1.5miles I guess?) to where your line of sight intersects the water, and from there the drop starts. I'll just go with a 1.4 mile advantage from the 2 foot camera elevation just to make an even 3ft. That means there was a drop over 3 miles from where your line of sight would intersect the water. A drop of 6 feet. 6 feet seems to correspond with what is shown in the aerial picture.