Comment 2 for bug 827950

dpkg isn't actually confused, it knows exactly what it's doing. When you build a package locally without using pkgbinarymangler, you wind up with full package changelogs, various documentation being file copies instead of symlinks, etc. When launchpad builds, it obviously applies all these changes to the package as part of the build.

So the contents of the packages are different between the two archs, and dpkg has no basis for deciding which contents are the "right" ones in a multiarch context. It does the only thing it can do, it refuses to install the new package whose contents differ from the existing ones.