Hi Chris,
The reason for this is you wouldn't usually defer the amount posted to the accounts payable GL code, you usually defer the amount posted to the expense.
Therefore if you are wanting to defer the amount to the expense you would post the journal like this:

and get GL entries like this:

Alternatively you could do this as a one line journal using the "Balancing Account". In this scenario you enter the GL code and deferral and balance to the Vendor:

This would give the same GL entries as the previous example.
Hope this helps.
Thanks
Gavin