Not sure why, because the two temp directories are different.
/tmp is usually purged during a reboot (at least you cannot expect to survive it)
/var/tmp is not purged during a reboot
The Qt::Dir() related stuff is actually a bug and should default to the wsutil function or at least the same g_get_tmp_dir, as Gtk and Qt might have different methods of getting the tmp directory and by using a Qt specific version this may lead to file polution.
I think using /var/tmp was a decision by the package developer for the Fedora package. I honestly see no reasons behind that decision, except to wanting to access the tmp files and making sure they stay around. But this leads to an overly crowded /var/tmp after some time in my book, so I think it actually should change back to /tmp
cheers
Roland