The less efficient your combustion chamber design, the sooner you need to start the fire.
Combustion isn't instantaneous - it takes time for the flame front to ignite the entire intake mixture.
The aim is to complete combustion so that the maximum 'push' on the piston occurs around 14° ATDC to achieve maximum torque at the flywheel.
The slower (less efficient) the burn, the sooner you have to start the fire.
Spark plug location and chamber design play a significant role.
A modern engine design e.g. 4 valves per cylinder with centrally located spark plug might only need 20° max advance because full combustion takes place quickly and efficiently.
Gene Berg acknowledged as much with his semi-hemi chamber mod - in one of his newsletters he advised running an extra 2 to 4 degrees of advance (e.g. 34° to 36°) when running semi-hemi cut heads. My interpretation of this is the flame front takes longer to complete full combustion due to the greater surface area created as a result of the semi-hemi cut chamber. Plus the entire squish area has effectively been removed, resulting in a less efficient burn compared to a more compact bath-tub style chamber with large squish areas top and bottom i.e. it takes longer to get the job done so you have to start sooner in order to reach completion at the same point.
The wasserboxer (in standard form) is another example of inefficient chamber design. The chamber in the head is very shallow with much of the combustion taking place in a large dish machined into the crown of the piston. At certain points in the engine's rpm range ignition advance can be as high as 50°
Compression ratio, deck height, bore diameter and type of fuel are also factors.
For the majority of Type 1 aircooled VW head designs, 32° total works out close to optimum but isn't necessarily the 'right' figure for everyone. It depends on all the above.
The best advance figure for a given rpm is the one that delivers maximum torque at that rpm. Having optimised advance for say 4500rpm, doesn't automatically mean that's the best figure for 6000rpm. The optimum advance setting usually occurs just before the onset of detonation. Peak power rpm (say 6000) might like more advance but can't be dialled in because that would result in too much advance (detonation) at peak torque. With a mechanical ignition setup you have to compromise.
Many aircooled VW owners run max advance in the sub 30° range. That's often because they're trying to compensate for something less than optimum about their setup e.g. too much CR, not enough octane etc but if that's what it takes to deliver best performance without the risk of detonation, that's what you have to run.
Alternatively, they may have a super efficient chamber design, run a tight deck, have plenty of squish area and relocated the spark plug(s).