A R-T element has no x-component to constrain. Consider
and for instance the basis functions:
Then you see that the basis function is vector valued. I.e. there is only three degrees of freedom, that are scalar valued.
u_h=\sum_{i=0}^2 c_i \mathcal{F}^{\mathrm{div}}\phi_i(x)
where \mathcal{F}^{\mathrm{div}} is the contravariant Piola mapping, see for instance: DefElement