Why is my journal posting in the HMRC VAT suspense account double counting the credit?

I have been posting our Postponed VAT as a journal, debiting the HMRC VAT suspense account with the PVA import 0% purchase on one side and crediting HMRC suspense with an equal amount with a Z (sales) vat code. This correctly places the vat in boxes 1 and 4 on our VAT return. We have noticed however that for some reason it also adds the credit to our sales figures, which is obviously wrong. We have been doing this for some time to get the correct figures in boxes 1 and 4 of the VAT return.