splitting Quantities while registering serial numbers

Dear Experts,

I have a question as I am new to AX regarding to splitting and posting quantities while entering serial number for items in PO.

when I try to register quantities with serial number, split the quantities to be 20 per each register, but when I select post all, I don't know why AX divide the quantities to be 1 unit per line, which means I have 40 lines as per following screen shots.

any one can explain the logic for this?

Regards,

Ahmad