These are because the help files are in HTML. Yelp search on HTML is currently flakey at best.
This is being worked on (along with improving searching generally). Upstream bug report: http://bugzilla.gnome.org/show_bug.cgi?id=347819 -- search results incomplete https://launchpad.net/bugs/50325 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
