Why some of the IDR invoices generating voucher number at the time of invoice creation instead of at

Why are some IDR invoices generating a voucher number at the time of invoice creation, instead of at invoice validation? Normally, the voucher number or document sequence should be generated during invoice validation. However, in this case, the system is assigning voucher numbers as soon as we import the invoices via invoice image, even when they are still in an incomplete status. Could anyone please clarify why this is happening?