Confused about mileage refund

I put in a claim for mileage for 22/23 (£1700) and 23/24(£1900) based on 45p per mile recently and got a cheque for £579 as they seem to do the calculations based on deducting these amounts on what I earned that year and recalculating tax amounts? Given i had paid the correct amount of tax prior to this claim according to HMRC - should I not be getting 45p on the mile back as I would if I could claim through employer? The 579 includes refund for £6 a week WFH so this would mean I got nothing back for my mileage? If

Hi Claire,
We would need to access your record to check the calculation.
You can contact HMRC at Income Tax: general enquiries for-individuals-pensioners-and-employees.
Thank you.