Today's arrestors separate water from the shock absorbing gas (air) by means of a piston or membrane.
Separation was not always the case. Arrestors were also once implemented by vertical capped copper stand pipes pre-filled with air. Unfortunately the air pocket's absorption capability would eventually fail as the pocket filled with water over time.
Let's compare the piston with the membrane.
The gas (air) chamber of a piston arrestors is set to about the water pressure during flow, and it is placed near the "culprit" valve (washing machine etc...). The hammer or shock absorption is accomplished by movement of the piston and back pressure from the trapped air that compresses behind the piston.
The required absorption depends on the kinetic energy of the flowing water, which is determined by the volume of flow. This in turn depends on the water pressure and the diameter of piping and valve.
Arrestors are therefore sized based on flow, and thus pressure and diameter. Buy the right sized arrestor for your case; pretty easy to pick for a home appliances, and requires a bit calculation to size for back to back washrooms, rows of urinals etc...
The gas (air) chamber of a membrane arrestor is set lower to account for the elasticity in the membrane and still achieve the same shock absorption.
And since the elasticity does not depend on the water pressure (but perhaps water temperature), a membrane arrestor is set with a constant offset.