First value aggregation weird result

Dear SAP Experts,

I have an issue regarding first value aggregation.

I have bex data like this:

Sales order# Material# Billing# Bill Amount Net SO Amount

S1 M1 # 0 6000 ->this row not yet billed yet

S1 M2 # 0 6000 ->this row not yet billed yet

S1 M3 B3 800 6000

S1 M3 B4 400 6000

Grand Total 1200 18000-> should be 6000

I have the aggregation to First value for Net So amt. and reference characteristic as Billing #.

Why I got 18,000 in total for net so amount instead of 6000.

If the first and second row not exist, I got the correct result :

Sales order# Material# Billing# Bill Amount Net SO Amount

S1 M3 B3 800 6000

S1 M3 B4 400 6000

Grand Total 1200 6000

Why I got 18,000 instead 6000 when I have null value for Billing# column?

Is there any solution for my issue?

Thank you very much.

Regards