why ext3 filesystem corrupt during graceful shutdown?

Issue

• The barrier test script available at http://lwn.net/Articles/283169/ is designed to introduce filesystem  corruption when power is abruptly removed from a server.  However, our tests reveal that running this script during a graceful shutdown (init 1) also introduces filesystem corruption on RHEL 5.5 using an ext3 filesystem.  This is true for filesystems on top of physical partitions as well as logical volumes.  Enabling write barriers (barrier=1 in the fstab) seems to have no effect on the outcome.  In all cases running e2fsck will show that the filesystem has been corrupted.
• why ext3 filesystem corrupt during graceful shutdown?