Correct.
If you rolled your 401(k) over to a traditional IRA in 2017, doing so has decreased the amount of your basis in nondeductible traditional IRA contributions that can be applied to the traditional IRA distribution and has increased the taxable amount of that distribution, even though the rollover might have actually occurred after the traditional IRA distribution. The unused portion of your basis remains in your traditional IRAs to be applied to later distributions. Your traditional IRAs will always retain some basis until your year end balance in traditional IRAs is zero.
