Shimano chains are considerably wider than SRAM or KMC as they have a different (Shimano-patented) joining system, where the rivets stick out and are supposed to be caught by the ramps and pins on the chainrings/cogs and give better gearchanges
- if I remember, a Shimano 9sp chain is actually about the same width as SRAM or KMC 8sp...
<edit> yes, chain widths are
Shimano HG 8 speed = 7.4mm wide
Shimano HG 9 speed = 6.6mm wide
SRAM 8 speed = 7.1mm wide
SRAM 9 speed = 6.9mm wide