I hope I am not asking a dumb question here, but I couldn't find anything like this when searching this site.
Here is what's happening:
I just noticed my site is listed in Yahoo, because my stats show someone found me. When I search for the particular keywords on Yahoo myself, my site is nowhere to be found.
I asked a friend if he could have a look and he told me my site ranks fourth for the keywords.
I then asked another friend to have a look, but my site doesn't show up when she performs the search.
Next, I went to one of those "check your Yahoo rankings" pages and that page told me I was indeed listed fourth for that particular query.
How is this possible? Is Yahoo serving IP based results? My friends and I are all in the same town in the same country, so IP-based results does sound strange to me. Any other clues?