Gross receivables and net receivables

Hello,

net receivables = gross receivables – allowance for receivables.
If we are given the net receivables at 31.12.20X1= 12.000 and an increase in the
specific allowance for receivables by $950 to $2.100 at 31.12.X2, why do we calculate the gross receivables at 01.12.20X2 (opening balance receivables X2) = 12.000+(2.100-950) and not =12.000+950? The net receivables at 31.12.20X2= closing balance receivables 31.12.X2 – 2.100

Thank you!