This looks like a bug which should have been fixed in the latest slackpkg version (which will come with Slack-11).
You can grab it here:
Just download the latest (2.08) version and upgradepkg.
(Note that the new "install-new" feature is meant to be used with Slack-current).
If it doesn't help, try giving Piter Punk a bug report (his e-mail is in the slackpkg homepage
From the changelog:
2006 Thu Aug 17
- Revert the "typo fix" and now "Formating" is "Formatting" again.
- A little fix in install-new to better handler the newer packages
- Big fixes in new priority check