First off, 10,000m is 6.2 miles, not 9.7 miles.
If the plane is 6.2 miles on the ground away and flying at 30K feet (5.7 miles), then basic pythagorean geometry says that a missile has to travel sqrt(6.2^2+5.7^2) = 8.4 miles, which is closer to the number they give?
First off, 10,000m is 6.2 miles, not 9.7 miles.