grub.conf and kernel updates?

[Tom Horsley]

Anyone know what exactly goes on when yum applies a kernel update?
Something automagically edits the /boot/grub/grub.conf file, but
whatever is doing that isn't doing it the same way on a couple
of different fedora boxes I have.

One box once had Xen installed, but I removed all the Xen stuff
because it wasn't very useful on the machine. This box now
inserts a new kernel at the top of the list of "title" sections,
but changes the default= parameter so the old kernel is the default
boot.

Another box never had Xen, and when it gets a kernel update, the
new kernel is inserted at the front, but the default= line is NOT
changed, so the new kernel is the default boot.

What the heck is different and how do I make them the same?