GT1955

Hi In previous years, I have had to make tax payments to HMRC of £5k plus each year. This year, my income has increased but my self assessment submission calculation is showing that the HMRC now owe me money. Can I just check, is this because the HMRC change individuals tax codes each year/during the year, or is there a potential error on my submission (which I have checked 2-3 times but appears correct)?