GOV>UK Personal tax account

I think could be because HMRC use "adjusted net income". To calculate your tax band and this includes savings income as it still regards it as income even though there is no tax to pay.

This means you can be considered a taxpayer by HMRC even though you have (legally) not paid a penny in tax!

At the thresholds between tax bands this becomes even more complicated. Imagine your income is £500 under the higher rate threshold but you also receive £1000 in "untaxed" savings income.

Many people would logically assume that as you are not required to pay any higher rate tax you would not be a "higher rate taxpayer", however in reality that untaxed interest would actually take you £500 over the higher rate threshold in HMRC's eyes, so even though the £1000 "would" have been tax free, by going over the higher rate threshold your savings allowance amount is reduced from £1000 to £500 meaning that your "upper" £500 of savings will now be taxed at 40% costing you £200!

I fell foul of this a while back where I assumed that "tax free" savings would be disregarded when calculating your tax band. I was wrong, and it cost me my £220 marriage allowance rebate because of it!