Gavra_at_work,
thanks for starting this thread! I just looked quickly at that gemara, not be’iyun, so if I’m wrong I’d appreciate someone correcting me.
There seems to be 2 separate things, a rubah d’leisah kaman and a statistical proof. The latter is called a chazaka or an umdena. The camel case is a chazaka not a ruba d’leisa kaman. Also, to answer squeak’s question, the case was that this particular camel was next to the dead camel so between the fact that it was right there and the fact that camels during that time normally get violent there is an umdena that this camel killed the other one.
A ruba d’leisa kaman is by definition a statistical majority, i.e. more than 50%, not a statistical certainty.
The gemara starts out on 92b with a machlokes between rav and shmuel if by financial issues we go by rov to force someone to pay. The case there is a ruba d’leisa kaman – when someone bought a cow which turns out be a gorer and the buyer says its a mekach ta’us and wants his money back. The seller says he doesn’t have to return the money because it is fit for shechita and that’s what he was selling it for (even though he never said so prior to the sale). Rav says the seller has to return the money because there is a rubah dleisah kaman that most people who buy cows are buying it to use it and not for slaughter, so if this cow was only suitable for slaughter the seller should have said so. Shmuel agrees that there is such a rov but says that the seller doesn’t have to return the money anyway because we don’t go by rov in dinei momenos.
Then on 93a the gemara wants to compare this to the case of the camels where its more than a ruba d’leisa kaman, instead its a statistical certainty.
The gemara wants to say though that the 2 cases would have the same halacha as each other, but then the gemara says that this is not necessarily the case. One can hold of ruba and still not go by chazaka in the camel case because over there there is also a rov that says not like the chazaka, namely squeak’s rov – that there are many camels in the world that could have done it so why say it was this one. Or you can not hold of rov and still go by chazaka because chazaka is more of a certainty than a rov.