Re-checking the form I now see what you mean. It may be that the 19.28 number is based on the reported sail area of 250 square feet, while the 16.90 number is calculated from the measurements taken from the actual spar dimensions and 100% of the foretriangle, which add up to 219.15 square feet. Drawings and pictures show that there is some overlap to the jib, so the two numbers would seem to reflect two different things. The one to use depends upon what you’re looking for. Actual practical performance based on the usual sail area of the jib and main (overlapped jib and probably some roach to the main, and so bigger, at 250 sq’) or theoretical performance based on just the triangular areas of the spars and forestay – smaller, at 219.15 sq’.