Overture & Yahoo - 3 or 4 listings?

Sometimes Yahoo lists 4 Overture links and other times its 3. Why?

[StephJurg]

Sometimes Yahoo lists 4 Overture links and other times its 3. I was wondering if anyone knew if there is any rhyme or reason for this. It seems to be pretty arbitrary. Sometimes its 3 links – sometimes its 4.

[kaz]

Not to state the obvious first to start your discussion, but sometimes there are only 1,2,3 bidders signed up for a particular keyword phrase.

[Jack_Frost]

More likely I think that it has to do on the more competitive phrases that have higher bids. I rarely see low bid, low volume search terms with more than three. Go to a phrase with 15,000 searches per month an 10 people bidding over $5 a click and I'll bet you'll see 4.

I have not spent the time to crack the formula, but it is interesting. There are many times where I just want to be in the top 3 so I make it to the top of Yahoo. If this term has 4 listings and the bid gap is nice, I can save a lot of $ and increase ROI.