Self Assessment online Calculator shows different results to when completing the actual return

Hi there, I am new to the self assessment tax return. In June 2022, I was promoted and received some relocation charges which tipped me over the threshold. All payments where tax deducted through the PAYE system. Moving forward, my annual income will be less that £100k. When I use the self-assessment calculator, the value of the total income tax and national insurance tax I should pay is more or less equivlant to the values that are on my 2022-2023 P60 form. However, when filing the online return, the calculated pending tax shows in excess of £1500. Please note that my tax code is 1257L. Could you please advise which tax amount is correct? Thanks for your help.

Hi Saqib,
If your income was over £100K in the 2022/23 tax year then this will affect your personal allowance. The personal allowance was £12570 for 2022/23 but if your income was over £100K for every £2 above the threshold if reduces your personal allowance by £1. If your employer operated the 1257L code giving the full personal allowance this will explain why tax is due. 
Thank you.