Trying to understand my repayment

Hello, Despite a long career as a self-employed person, is my first year filing self-assessment without an accountant. I have already filed my return for 23/24. From my workings, I understood that I was due a repayment around £2,600. However, I've already received the repayment and it's around £1,900. Is there something I've not figured in to the repayment? Where can I find out how it's worked out? I simply worked out tax paid on account minus what is now due. There doesn't seem to be any error with the tax I've said is due. Many thanks in advance.

Hi Cityseaedge,
To find out how your repayment has been calculated -
Sign in to your online account. 
Select the option to view your latest Self Assessment return.  
Select ‘View statements’.  If you are still unsure, please phone us on 0300 200 3310 and we will explain it over the phone.
Thank you.