Bill In arrears Invoice accounting DR the Receivable from the first period and credits the revenue

1- an invoice is entered with invoicing rule In Arrears.

2- variable schedule of 3 months,

3- revenue recognition program is submitted.

4- create accounting resulted in

   a- Period 1:  Dr Receivables 90

                        cr Revenue        30

                       CR Unbilled receivables  60

b- Period 2 : Dr Unbilled receivables 30

                    cr Revenue 30

c- period 3 :  Dr Unbilled receivables 30

                    cr Revenue 30

Why The receivables is recognized from the first period ???