I can confirm this bug. I have the same gthumb version and package. Playing around with this, it looks like sloppy string matching. I reported it to the gnome bugzilla:
Gnome bug #346382 http://bugzilla.gnome.org/show_bug.cgi?id=346382 -- find by category returns false positives https://launchpad.net/bugs/48922 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
