I think there is a bit of a misunderstanding here. The chip's datasheet
indicates a range of inductance over which the chip's built-in varactor
can successfully bring to resonance. This is neccessary, but not
sufficient for calling the antenna "optimal". The choice of which
inductance works "best" overall probably depends on several physical
factors and usage patterns. But once chosen, the chip will
automatically resonate the inductor, which eliminates the factory
alignment procedure which has historically been needed. I don't think
the inductance spec should be read as indicating that all inductances
will perform equivalently; only that all within range can be resonated.

As for the chip tuning inductors outside the specified range, please
recognize that a datasheet is a guarantee of performance. In the case
of integrated circuits, it is critical that a vanishingly small number
of them fall outside the datasheet parameters when deployed in the
field. No manufacturer can afford to scrap complex devices when one of
many chips inside fail to perform as specified. So the numbers in a
datasheet are all conservative, some are very much so.

-Scott-