DRV8889: Unexpected voltage drop at output
There are no datasheet high/low limits given for the Kv parameter- only a typical value. It will vary from unit to unit, and with temperature. 

Resistance of the winding may not be exactly nominal and will vary considerably as the coil heats up (about +0.4%/K).

The current mirror approach may lose accuracy as the output voltage approaches the rails.

Your reference voltage may not be exactly as predicted - measuring it directly will reduce the uncertainty, but ground currents from the motor will affect it a bit, worse if the layout is not ideal.

So there are plenty of opportunities for differences from the ideal, and I don’t see a way to estimate the potential Kv variation.