portupgradeのフシギ

例えば、ports-mgmt/tinderbox-develを以前にインストールしているとする。そして、ports-mgmt/tinderbox-devel/Makefileに

IGNORE=         please use ports-mgmt/tinderbox instand

とあると、portupgradeできないわけだ。しかし、

$ sudo portupgrade -f -o ports-mgmt/tinderbox ports-mgmt/tinderbox-devel
** Detected a package name change: tinderbox-devel (ports-mgmt/tinderbox-devel) -> 'tinderbox' (ports-mgmt/tinderbox)
--->  Upgrading 'tinderbox-devel-3.2_4' to 'tinderbox-3.3_3' (ports-mgmt/tinderbox)
--->  Building '/usr/ports/ports-mgmt/tinderbox' with make flags: DISABLE_VULNERABILITIES=yes
===>  Cleaning for tinderbox-3.3_3

===>  tinderbox-3.3_3 conflicts with installed package(s):
      tinderbox-devel-3.2_4

      They install files into the same place.
      Please remove them first with pkg_delete(1).
*** Error code 1

Stop in /usr/ports/ports-mgmt/tinderbox.
*** Error code 1

Stop in /usr/ports/ports-mgmt/tinderbox.
** Command failed [exit code 1]: /usr/bin/script -qa /tmp/portupgrade20100104-86319-16fdo3h-0 env UPGRADE_TOOL=portupgrade UPGRADE_PORT=tinderbox-devel-3.2_4 UPGRADE_PORT_VER=3.2_4 make DISABLE_VULNERABILITIES=yes
** Fix the problem and try again.
** Listing the failed packages (-:ignored / *:skipped / !:failed)
        ! ports-mgmt/tinderbox (tinderbox-devel-3.2_4)  (unknown build error)

となるのはどういうわけだ?

/usr/local/etc/pkgtools.confでMAKE_ARGSにDISABLE_CONFLICTS=yesを追加すると、このエラーを回避できるが、何か間違っている気がする。