Courtesy of a wonky prerm script in a package I'm working on, I just learned that when you dpkg-reconfigure a package, it runs more than just the package's config script. I guess subconsciously, I'd always expected it to run the postinst again as well, but I did not realise the prerm script was also run.
So having now peeked at the innards of dpkg-reconfigure, it runs the prerm with upgrade version, the config with reconfigure version and the postinst with configure version, in that order.