The more likely story is, that Apple set some initial specs for the face ID module early in the development. This was probably a very strict spec, which is of c

The more likely story is, that Apple set some initial specs for the face ID module early in the development. This was probably a very strict spec, which is of course difficult to match. Fast forward, some months later, as the production devices are coming together and real production devices can be evaluated, the specs can be relaxed somewhat, as Apple now has a clear idea about the specs they need to have a functioning face ID device. Also, the software might be a bit more tolerant to variations in the device. Of course, the revised specs will increase yield too. But that is all what engineering is about -finding out how far you can relax specs and still have a robust working device.