Not understanding why the white dial measures 12.9 to the 12.6 of the black dial? Unless the white dial is ARF, which I doubt. This is where speaking mandarin would come into play.
+/- 0.3mm could easily be the difference in assembly tolerance between different pieces from the same batch.
The assembly compression on crystal + caseback gaskets isn’t precise on reps. This alone could alone explain the difference in measured thickness.
Remember also that the machining tolerance on the mid-case could easily be +/- 0.05 to 0.1mm. Then the caseback tolerance could also be +/-0.05mm. I’m sure the gasket thickness also varies. I don’t know, I am just estimating the machining tolerances for illustration purposes.
So All these tolerances add up.
China tolerances are understandably not the same as Swiss tolerances.
Hence explaining why there is always some difference in thickness between pieces.