I can't see it either.
50.29*19.51 (entire block) is about 981.
Inner block (say 30*14) is about 420, leaving 561 for the outer block.
My rough calcs of driveway (5*30 = 150) plus bulk (19.5*(50-30) = 390) give me 540. (OK, so I'm out by 20m2, but it's rough, alright?)
As the say in the classics - "please explain"