Personal Tax Allowance reduced even though I earn below the limit

Hi, My personal tax allowance has been reduced to £9,460. I earn £7,500 from a private pension which is way below the £12,750. When I queried this via a phone call to the tax office, I didn't get a clear reason why, but they were happy to change it back to £12,750. However, they also stated that they'd lower my tax code every year and I'd have to keep calling back to have it raised again. This appears to be a needless waste of everyone's time as I should just be given the code associated with the allowable personal tax limit as a default. Can someone explain why I will have to call back again next year? Thanks

Hi Mike_the_Engineer,
We would need to access your tax record to check why the tax code was amended.
You can contact:
Register for Self Assessment if you are not self-employed
Thank you.