GSB

I am trying to understand how my tax code is calculated. I have a personal allowance of 12700 and a state pension of 10500 so my tax free amount is 12570-11500 or 1070. A recent notice about the tax code says in Note 4: "To create your tax code we have removed the last digit or your tax free amount and added a letter". So why is my tax code 108, not 107? Talking to a friend, they too have a discrepancy of 10. What else is in the calculation, or is this an extra, early, Christmas bonus?