P60tax differs from Self Assessment return

I have only earned employment Income in 2022-2023. My employer correctly used the issued tax code to deduct 2022-2023 tax. In addition, the HMRC tax calculator arrives at the same tax as my P60. However, when I complete my self assessment, it is showing a higher tax and that I have overdue tax. I cannot get through the phones as the AI tool rejects to put me through to an agent to enquire why I should pay the additional tax. Can anyone help?

Hi Aleqs77,
We would need to review your calculation to check the reason for the balance being due.
It could be if your income was over £100,000 and the tax code used by your employer gave you the full personal allowance then as full personal allowance not due this would cause an underpayment.
For every £2 above the £100,000 threshold it reduces your personal allowance by £1.
If this is not the reason you would need to check the enteries made on your tax return are correct and if so submt the tax return.
If you then allow 3 working days to process to HMRC records and then contact us to review:
Self Assessment: general enquiries
Thank you.