Why does MATLAB's dualtree3 function not return the lowpass subband of the imaginary tree?

MATLAB has a function named dualtree3 which computes 3-D dual-tree complex wavelet transform. This function only retains the last low-pass coefficients/subband of the real tree. And, it does not save the low-pass subband of the imaginary tree. How can you justify this?

Previously, I have also asked this question here.