Same tax classification but tax is generated differently

Hello

Could you please advise the issue below?

I have 2 invoices, one is non-PO invoice and the other one is PO invoice.

Tax classification code on invoice line is the same e.g. REVCHG20 and supplier is the same for both invoices.

The issue is with PO invoice that we are reported that when calculate tax, the tax that generated is wrong. It generates GB-OFFSET which it is not what user expected.

What they expect is GB-REVCHG tax which happened to Non-PO invoice, when calculated tax, GB-REVCHG tax is generated.

See screenshot below for Non-PO invoice and PO-invoice.

I don't understand that with the same tax classification code, same supplier, why tax is generated differently. Any advice on how GB-OFFSET generated please?