is there a bug with the exactarch option?

Hi,
I'm running Scientific Linux 6.2 x86_64 (Redhat 6.2 clone) and I also manage a
repo for it, which has both i686 and x86_64 packages in it, that aren't always
exactly at the same version.
The repo is here:
http://pkgrepo.linuxtech.net/el6/release/

As you can see the package ffmpeg-libs for example is at version 0.6.3-2 for
x86_64, while it's already at version 0.6.5-1 for i686.

What happens now is that on the x86_64 box where I only have x86_64 packages
installed so far (including ffmpeg-libs-0.6.3-2) if I run 'yum update', yum
will try to use the newer i686 package (ffmpeg-libs-0.6.5-1) to update the
installed older x86_64 package.

This happens despite I have 'exactarch=1' in /etc/yum.conf.

Isn't 'exactarch=1' supposed to prevent this, or am I misunderstanding the
purpose of 'exactarch=1'?

If 'exactarch=1' isn't supposed to prevent this, how else can I avoid this, as
 upgrading a x86_64 with a i686 package will obviously cause a mess and
therefore shouldn't happen?

Scientific Linux 6.2 uses yum version 3.2.29.

Thanks in advance for any clarification.