I hope I understand this right.
A straight wire runs through the sensor, and a hall sensor picks up it's magnetic field.
The sensor outputs exactly [u]half[/u] of it's own [u]supply voltage[/u] when no current flows through the sensor.
Sensitivity is how much the output voltage deviates from 50% supply.
The sensor can pick up DC (both directions) as well as AC.
It makes sense that you supply it from Arduino's 5volt line, so any Vref variation still reads 512.
Since ~5mV is one digital step, 1A should deviate ~13 digital steps per Amp (if 66mV/A).
edit. How do you meaure a PWM speed controller.
Current draw is not constant.
If you take a sample at the wrong moment, you could be measuring nothing.