Updated News: Yahoo Publisher Network in International Markets

It isn't pure conjecture, it's in their TOS. Yahoo is paying a higher than normal percentage during its beta to get new publishers (promotional payout). Even if they paid 100% with no cut, it still (obviously) depends on what their advertisers they have are willing to pay - and this amount, like adsense, can vary daily because prices are determined by a bidding process. Sometimes one advertiser can drop out and cause a pretty large drop in a shallow pool of advertisers. As adsense has so many more advertisers, they are less likely to have a large drop because of their ad depth.