A bit off topic, but there is a beautiful, free publication from Klippel about flaws in loudspeakers, called Loudspeaker Nonlinearities – Causes, Parameters, Symptoms . The measurement and analysis tech has gone a long way to have a lot of insight into inner workings of a driver. It is DEEP. The same tools are used to analyse headphone drivers and microspeakers so I guess it's not entirely irrelevant.

The best drivers are optimised with software and a ton of simulation of the magnetic circuit and the airflow around it. The experimental research is carried with simple and complex signals. Not as complex as the ones from Jerobeam Fenderson, but still it's more than what you see on ASR.

They dissect distortion into many parts : HID, IHD, IMD, AMD and some more. They scan the entire diaphragm with a laser and then split the recorded movement into discrete components so that you can see if your diaphragm is making sound or just rocking.

Generally the task of a headphone driver is to turn voltage into pressure just like a compressor in your fridge, except it needs to happen on time. When voltage changes by one unit, the pressure must change proportionally and under varying conditions. Some drivers do it well, some don't. The 70 page paper I mentioned describes the issues that transducers face. A lot of it comes down to quality control. The symmetry of the assembled driver is not always the same. This last thing will become less of a problem with MEMS microspeakers, but with a driver assembled by hand there are always rejects that end up on Aliexpress. You can buy a driver and see what's inside. You'll see that there are many different drivers, made with different materials, vents, rings, magnets. Their coils have various diameters and lengths. You'll also see their prices.
If you go on ASR, you'll see that they also measure differently.