When dealing with building materials, notably tile, you have to make allowance for unusable scrap when the floor dimensions do not come out to an exact number of tiles and you have to cut some to fit. This calculation is "unscientific", some tile makers give an arbitrary fudge factor such as "order ten percent more".
Recently I did an insulating subfloor where I did a "sceintific" calculation of how many pieces would be needed. Because this would be covered with a (more attractive) finish layer, I could stagger the joints/seams in an uncanny fashion to make better use of "scraps".