The "identical hardware" assumption is where your argument falls apart.I agree with you. The identical hardware is in every device and I doubt there are that many where something was done in assembly to cause them not to work.
This may be news to you, but every single component in the phone will be manufactured to a drawing or specification with tolerances on it.
That includes the sensor components, so there will be inevitable variation in sensor performance arising from component variation.
That's how engineering works.
If you don't understand that, read up about manufacturing tolerances and statistical process control. Then go and do a 6-sigma course, preferably followed up by a 6-sigma project.
Applying normal engineering principles:
1 Even if the sensors all come out of the same factory, there will be variations in sensor performance. That's due to component tolerances.
2 Unless we have access to the Google BOM and supplier sourcing data, we cannot even assume all the fingerprint sensors come from the same supplier or are manufactured to the same design.
That's before we even talk about engineering the sensor to work under a reasonable range of environmental conditions (humidity, temperature etc), for different size fingers and other variables.
There would be an awful lot of engineering required to do this properly.