Why is --replacepkgs needed?

[susan_geller at speakeasy.net]

I was having some trouble with various audit messages after I upgraded to a newer version of the selinux-policy-targeted rpm.  I did
rpm --verify selinux-policy-targeted
and found that several of the files didn't correspond to what is in the rpm.
So I did
rpm -Uvh --replacepkgs ...
and everything magically works.

Why can't the rpm packages be made so that the --replacepkgs flag is not needed?

Thanks, Susan