[Wlug] inconsistent results with du and multiple arguments

Can anyone explain why du (which advertises the ability to evaluate N file/dir arguments) seems to only be accurate for the first provided argument? See the following: brett@spider:/> sudo du -smxc tmp 60 tmp 60 total brett@spider:/> sudo du -smxc tmp bin 60 tmp 1 bin 60 total brett@spider:/> sudo du -smxc bin 9 bin 9 total brett@spider:/> sudo du -smxc bin tmp 9 bin 1 tmp 9 total