Its because of most company policy of limiting the increment of new salary based on old salary.

I had face this before, where during interview, we had agreed on my salary. Then when HR reach out and asking for payslip, HR reject what was agreed with interviewer as the increment is too much.

In the end, I get 30% lesser salary than what was agreed, but interviewer willing to compensate the remaining 15% with allowance, and I have to let the remaining 15% go.