9 speed chains are only a little narrower internally
No, they have shorter pins than 7-speed chain too, by over 1mm.
There has been a steady progression in pin length, but only one jump in roller width which was at the 8 to 9 speed transition. The first "narrow" chain was Ultra6, used to squeeze 6 sprockets into the space previously allocated to 5 on 120mm hubs, followed by Ultra7 using the same chain fit 7 sprockets on 126mm "6-speed" hubs, and each successive increase in number of sprockets has required a further narrowing of the pin length.
No, they have shorter pins than 7-speed chain too, by over 1mm.
There has been a steady progression in pin length, but only one jump in roller width which was at the 8 to 9 speed transition. The first "narrow" chain was Ultra6, used to squeeze 6 sprockets into the space previously allocated to 5 on 120mm hubs, followed by Ultra7 using the same chain fit 7 sprockets on 126mm "6-speed" hubs, and each successive increase in number of sprockets has required a further narrowing of the pin length.