Assuming that you had investment gains in your Roth 401(k), it's likely that only some of the amount rolled over to your Roth IRA, the amount shown in box 5 of the Form 1099-R reporting the distribution from the Roth 401(k), became contribution basis in your Roth IRA. Unless your distribution from the Roth IRA is a qualified distribution, only the amount of your Roth IRA distribution up to your contribution basis is tax free.
(Don't refer to the movement of the funds from the Roth 401(k) to the Roth IRA as a "conversion." The term 'conversion' is used to refer to a distribution from a traditional IRA that is rolled over to a Roth IRA.)
