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