I converted $2310 from my non-deductible IRA to Roth in 2021, why is part of it taxable?

In 2021 I converted $2310 from my traditional IRA to Roth IRA. I only have one IRA account and I've been making non-deductible contribution to this account since 2013. I received 1099-R this year with Distribution Code 2. I entered it into TurboTax Deluxe Desktop. It tells me only $519 is non-taxable. I don't understand why. Thanks!