Yes I understand, since I wrote that I believe that Madara's Susanoo was incomplete to a point, because:
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This is Itachi's incomplete Susanoo, it consists of just the layer of skeleton and nothing else.
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Here we can see Itachi's incomplete Susanoo
forming into that of his completed version, there is now a attached flesh layer to the right side of his body.
Now onto Madara
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This picture explains everything to me, yet it confused me as well. Madara's rib cage is still clearly visible, however the upper half of his Susanoo has got flesh all over. His hands haven't got any visible bones nor does the arms or the shoulders.
Looking back at the complete Itachi Susanoo:
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We can see everything, including his rib cage is now covered in the second layer of flesh, meaning it is fully completed, however not yet armored.
So what I believe now is, and it may sound quite radical, but Madara hadn't fully formed his Susanoo, but only to that where it can perform essentials hand seals in order for Madara to perform
''That Jutsu'' and
Yasaka Magatama. Which is why he left the Rib cage visible and not covered in flesh, he tested the Kage's power, by lowering his defense yet keeping his offense to a level where he could also attack back to persuade the Kage's that he is fighting them back.
There is much more of Madara we have yet to see...