seems pretty obvious. the aircraft at 1,000ft spends less time in the climb and more in straight and level cruise so must cover the greatest ground.
circumference is pi times the diameter
so a circle at 1,000ft has a circumference of 12,742,304.8m x pi = 40,031,131m
and a circle at 40,000ft has a circumference of 12,754,192m x pi = 40,068,476m
not that much difference.