DRV8825: Why the actual current is always lower than the set-up current?

Part Number:DRV8825

Both of  My current sense resistor is 0.2 om,  and my AVREF an BREF is  set to the same value.

So,according to the calculation formula， the current chopping threshold  for the maximum current step   I(fs) = Vref.

 I  set the Vref (pin12 and pin13) to  0.13V,  when the step motor stoped,   the actual current is 0.12A.

 I  set the Vref (pin12 and pin13) to  0.24V,  when the step motor stoped,    the actual current is 0.13A.

 I  set the Vref (pin12 and pin13) to  0.41V,  when the step motor stoped,    the actual current is 0.15A.

 I  set the Vref (pin12 and pin13) to  0.70V,  when the step motor stoped,    the actual current is 0.18A.

Why  the actual current  is always lower than the set-up  current?