However, in most parts of the country, days above 90 degrees are uncommon. The researchers neglected to control for this fact. Because each of these numbers have zero deviation from the average, including them will decrease the average squared deviation. Including these numbers will increase the average squared deviation. Using the normal approximation, the percent of men within 1 SD of average is roughly 68.
Men tend to weigh more than women, so combining the two groups results in more variation.
The combined SD will be greater than 9. It follows that the girls must have averaged more than 147cm, so on average girls are taller than boys at age 11. Boys averaged 136cm at age 9 and 146cm at age 11. Absent any additional information about growth rates, we estimate the average at age 10 to be halfway between 136cm and 146cm, i.e. 141cm. Something is wrong with the computer.