Maybe it is just my limited production knowledge, but wouldn't it be possible to injection mold a bigger part and then mechanically shave off the last few fractions of a millimeter using any number of ways? Tooling costs too high. But in the simplest form you could essentially spin the fan against some adjustable abbrasive to shave off the final bits.

Granted, there may be other places in which the molding precision may matter, which would make this an impractical solution.

Okay. Seems like low noise is another big customer draw. So what's the difference for those measures between this difficult to manufacture fan and one with clearances that are easier to manufacture? If either is particularly significant, it's quite a bit more interesting than the measurement of the clearance.