I found it weird too. Actually I have similar statistics as you do.

To test, I just tried searching by Yahoo! a few really old threads (but very specific in topic) I posted a few years ago, by using a few very specific keywords. Both Yahoo! and Google got the threads. So it seems like the threads have actually been crawled. They just do not turn up early in the search results unless I am using very specific keywords. This seems to especially the case if I use Yahoo!.

For my personal site, awstats reported 1294 referrals from Google and 68 from Yahoo! for the month of June. For the keyphrase "perl tutorial" that gets the most referrals for me, Yahoo! turned up my site on the 6th page while 2nd page for Google. I did not perform any SEO intentionally.

Well, of course, if a search result appears on page 5 or later then we can safely assume nobody is going to see it ...