I've installed a package that is hosted by multiple archives.  In this example, it's "dirvish," a nice dired wrapper.  It is hosted in both non-GNU ELPA and MELPA.  The dirvish author asked for feedback on the MELPA version before bumping official version numbers which update releases on ELPA, hence I have the hand-selected MELPA dirvish installed.  I have package-archive-priorities that put both ELPAs above MELPA.

When I call package-reinstall on dirvish (to overwrite local changes, for example), the non-GNU ELPA version is installed rather than reinstalling the MELPA version which is what I expected to happen.  I'm guessing most users have the same expectation.

-Stephane