Why can the draught amidships not be used to extract the value of displacement from the displacement table?

The ship is box that is never symmetrical about its central axis as built

Also due to various drafts tythe waterplane in real differs from theoretical waterplane

Hence the 6 points drafts ,read at stem, midship and aft ,are first corrected to Length between perpendicuar , then list and trim , brionging the mean hydrostatic draft very close to tabular draft of designed ship

mere mean drafts could indicate higher submergence due to sag ( loaded heavy in middle ) or hog( loaded heavier in end holds)