Smart shunt drift after 16days with no calibration. Who is to blame?

We're charging the battery to 3.45V again and the smart shunt should show the same SOC on the display as 16 days ago. But it shows 2% less .
Is this a problem with the smart shunt accuracy? Have the cells really drifted because of different internal resistance and therefore different heat loses? Or is the SOC inconsistent at 3.45V as we have seen in our test recently (link below)?

Inconsistent charging to same voltage:
   • Finding 20% and 80% SOC with LiFePO4....  
Here is the spreadsheet and graphs for this test:
https://bit.ly/2VYuol1

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