Unable to understand the threshold calculation for payables withholding tax invoices

The calculation point is set to Payment. We created two invoices — TEST GROUP 1 and TEST GROUP 2.

TEST GROUP 1 has an Exempt tax rate and amount is 3000

TEST GROUP 2 has a Non-Exempt withholding tax rate of 7% and amount is 25000.

Threshold Bucket Level - Party
Threshold Basis - Period
Calendar - Annually
Threshold Type - Taxable Amount
Min Value - 1500
Threshold Taxable Basis - Amount in excess of minimum value

When the two invoices are paid one after the other, the tax rate shown for TEST GROUP 2 appears as 7.42% and withholding amount is 1855, and the calculation seems to be based on the combined total of both invoices. We are not able to understand why the exempt invoice is also considered for the second invoice withholding calculations.