It is a stupid hidden tax thing. They don't tax the first 12.5K of income, if it is below 100K. They start reducing that 12.5K limit as your income goes up, for every £2, it reduces by £1.
So, if you earn 100K, the first 12.5K of your income is not taxable
If you earn 105K, the first 10K is not taxable
If you earn 125K, there is no non taxable amount.
So the tax rate gets applied two times, that's why the strange thing between 100K and 125K happens