bq40z60: RSOC not reaching 0% ?

Hello,

I have a bq40z60 gauge that is monitoring a 4S1P Li-ion pack.  I have calibrated the current, and checked that cell voltage is accurate. The ITEN register is set.

After several charge/discharge cycles, the RSOC still does not reach 0% before cell under-voltage (3000mV) is reached.  This occurs at between 40% to 20% rsoc, and it varies every cycle.

Is there a procedure for calibrating RSOC? 

Thanks