vocoder output

[jbv]

I have a question :

Looking at the synthesizer section of the block diagram
of the ETI vocoder, I noticed that the outputs of bands
3,5,7,9,11,13 are sent to an "inverting mixer" before
being fed into the final output mixer, along with
outputs 1,2,4,6,8,10,12,14.

Why is it so ? Is there any phase problem between
contiguous bands ? And why is it different in the
Elektor vocoder, in which all bands outputs are sent
to one single mixer ?

jbv