Why do the speeds of two ceiling fans vary on all speed settings?
Motors don’t always spin at the same speed even if the same brand, model and horse power.  They may have different speeds. I support a 1200 horse wood chipper with 3ea 400 horse motors. To get this monster to run and have the system balanced the motors need to be running at the same speed. we had to add variable frequency drives and match the full load speeds and current draws, so in short It could be the motor or the speed controller is not calibrated the same on both motors but fans usually have multiple windings for speed control and they can and usually are slightly different I see +- 50 rpm difference on motors rated at or around 1750 RPM. The one exception would be synchronous motors these are usually dead on and tied to line frequency.