After some more reflecting, I think that the root of the problem probably may be with the remote zypper repository. I manually create the delta rpm's my self and copy them into the zypper repository.

Is the filename of the delta repository important? I hope not. Or maybe it has to have a .delta.rpm extension. Dunno.

I create the zypper repo with:

# createrepo .
where . contains both full rpms and delta rpms.

Is this correct?

When I look at my SMT repos, I see that the repodata contains a deltainfo.xml.gz. How did that file get created by SUSE? I can't get createrepo to generate a deltainfo.xml.gz for me.