IEC 61000-4-2 requires 6kV for contact discharge at the specified test levels. Why is there no 6kV requirement for air discharge?

For contact discharge, the test voltage and peak current have a linear proportional relationship; however,

for air discharge, as the test voltage increases, the peak current reaches a plateau,

2kV⇒4kV⇒8kV⇒15kV, thereby establishing relationships of 2 times, 4 times, and 7.5 times.