The reason foods have these instructions is because the boiling point of water changes with altitude. As you go higher, the boiling temperature decreases. As a general rule, the temperature decreases by 1 degree F for every 540 feet of altitude. So pasta or potatoes cooked at sealevel are seeing 25 degrees more heat than pasta or potatoes cooked on Pike's Peak. The lower heat means a longer cooking time is needed.
Pressure cookers work in the opposite direction.
A pressure cooker raises the pressure, so water boils at a higher temperature. The higher temperature means that foods take less time to cook. Inc. All rights reserved.