I am going to make a guess.

The assumption is that when you have a Head Gasket leak there is a pocket/s of gas somewhere in the system. So since gas is compressible even if the gas pressure exceeded the Radiator Pressure Cap pressure and vented off some that would still be compressed gas in the system.
When the Engine has cooled off the gas volume has decreased but is still pressurized and I believe it would hold that Vacuum Valve in the Radiator Cap closed. So in this case your Hoses would remain inflated; so to speak.

So the compressible gas volume acts differently than the uncompressible Coolant.

When the Coolant volume decreases it opens the Vacuum Valve and decrease the system pressure.

However, a low Coolant level that has an Air Pocket may be able to mimic a Head Gasket problem by haveing the had Radiator hose.

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84 300D, 82 Volvo 244Gl Diesel