As you are pumping the tyre up. The volume of air gets smaller, so the pressure increases. When you have the air out of the pump & into the tyre the reading will be lower than any visual pressure reading taken whilst pumping the tyre up. More common at the lower pressure.
Sub Aqua
Are you saying you are taking gas into your lungs at 140lbs a square inch?
Used one that was pumping air out at over 400 lbs a quare inch. That had no resevoir on it though. I did say that they work on a short stroke, wide bore cylinder. The end result being frequent, high pressure bursts. Well above that used in any bike tyre. Steel also holds up better than a thin rubber tube to high pressure.