Ok.. let's look at some facts before making any explanation:
Assuming atmospheric pressure at sea level = 1 atm then:
fraction of 1 atm average altitude (m) (ft)
1 0 0
1/2 5,486 18,000
1/3 8,376 27,480
1/10 16,132 52,926
1/100 30,901 101,381
1/1000 48,467 159,013...