The result implies that either you have not entered the traditional IRA contribution as being nondeductible or you indicated to TurboTax that you had a non-zero year-end balance in traditional IRAs. With a nonzero year-end balance in traditional IRAs, the taxable amount of your Roth conversion is determined on Form 8606 Parts I and II. With some amount of your Roth conversion being taxable, some amount of your basis in nondeductible traditional IRA contributions remains with your traditional IRAs.
