How does the number of floors affect cost beyond just multiplying the footprint?

Additional storeys increase vertical transport of material and labour, and they can require thicker structural members at the lower floors to carry the extra load, so cost does not scale in a simple straight line with floor count. A G+2 house is not simply three times the cost of a single ground-floor footprint; foundation and lower-floor columns are designed for the full eventual load. This is one reason the calculator treats storey count as its own input rather than a multiplier on area alone.