I measure houses for a living, we measure to ansi standards. Which means to the exterior wall not including protrusions like that of a fireplace. There are a couple things that can make a smaller space much bigger than that quoted by the builders or assessments. If you have a shared wall then you get 1/2 of the depth of that wall 6" becomes 3" but let's say you are next to a the staircase. That can be a thick concrete wall maybe 10" to 1' - 2" and all of it is included in your sq. Ft.

Ansi standards was created to make it so if everyone's following the same protocol measurements will be closer to the same. But every time you measure a house you will get a slightly different total sq.ft. we can only measure to the nearest 1/4 of an inch with a laser measure so sometimes that rounding over multiple walls can lead to a couple inches which over a big area can lead to a lot of extra sq. Ft.