Never noticed this before - I knew ZX6r carbs had different main jets for the inner / outer carbs but never realised they had different needles as well -

http://www.kawasakioriginalparts.com/kawasaki_motorcycle_parts_selection_pfk.php?block_01=E1612&block_02=99917-1825&block_03=MC&year01=199 9&ca=GB

I wonder what difference is? Can you tell by looking if the needle profile is different?

I found the zx9r E carbs have different size main jets.
I was told the reason is some thing to do with the two outer cylinders been colder then the inner two when in a bike frame .
I don't know if this is true but its what i was told
Jacko