[jonahx]

A possible interpretation....

Using the insight from the top comment that "it all depends on whether u're counting the items themselves (1-based) or the spaces btwn them," the American way of numbering floors is based on counting actual floors (ie, the things you stand on) -- and the one at earth level is one floor. If you go up a flight of stairs, there is second floor to stand on, and so on.

For buildings that go underground, the "-" sign can now act as a signifier of being underground, and the counting works as normal. If you take the stairs down one level, you are on the first underground floor, -1.

Of course, you want to interpret it like a y-axis number line, where 0 is the earth, 1 is "1 floor unit" above the earth, -1 is "1 floor unit" below the earth, etc. This is the "space between" model.

Elegance aside, both can be viewed as logically consistent depending on your lense.