That does not imply it is abnormal to remember overall number after you visualized it few times. I would even argue it is abnormal to not remember result of calculation you have done multiple times.
It's a useless number. People don't tend to remember useless numbers they can recalculate with no effort. Same with the number of doors or plants.
And they also don't need to visualise whole house to get that number. That strikes me as the more absurd way of finding the number even when you are calculating it.
Can't argue with that. It's just easier to visualize a complete city with all the houses inside of that city than it is to remember a useless number. At least for me. Text is hard for me to remember, pictures aren't.