Because nothing is TRULY incompressible. Any liquid or solid will be squished a bit under pressure. IIRC water density increases by a few tens of a percent under tens or hundreds of atmospheres. You have to be doing something hyper specific for that difference to matter.
In comparison, you can decrease the volume of air a few hundred times, until the gasses eventually turn to liquid themselves under pressure. The tiny CO2 canisters used in airsoft hold 60 liters of gas, for example.
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