Well you're right, there is only one pressure, but can be called different things. Yours will be gauge pressure. With the pump off and pressure bled off it will be zero, atmospheric pressure is read as zero on a gauge pressure, pressure gauge. A standard day atmospheric pressure is 14,7 psi. So if you were to measure your fuel pressure in absolute pressure it would be your 59.5 plus 14.7 for 74.2. But normally we measure the pressure of fuel, air in your tires, oxygen in your tank, MIG gas in your welding equipment, what you run your air tools at etc. at gauge pressure. When the power goes out and your impact wrench drains the compressor tank, or you run over a plow blade and your tractor tire loses air the pressure on your compressor gauge or your tire pressure gauge will be zero. Even though all around us the air pressure is 14.7psi. When someone tells you to fill your tires to 32 pis it's just automatically meant 32psi on your tire gauge, 32psi higher than atmospheric, not 32psi absolute. 32psi gauge is the same as 46.7 absolute, but called two different things.