Nachmanides asks in the first mishna in Ketuboth this. We know that if the husband comes to court and says he found his wife not to be a virgin we believe him until she brings proof that she was. [So she loses the Ketubah and also to one opinion the marriage itself is not valid. [It is like when you buy something and what you get were two different things. The whole deal is null in the first place.] ]

The Gemara says the reason is that the Ketubah is from the words of the scribes, not from the Torah. So if it would be from the Torah she would be believed? And he would need to bring proof? But why? In general when a document comes to court that has a condition stated in in it do not we usually say first show that the condition has been fulfilled and then we can deal with the document?

The Ramban [Nachmanides] says the reason she would be believed is there is a Hazaka and a Rov [prior status and a majority that most women are married as virgins.]

So we see a hazaka (status) alone would not be enough. Rav Shach shows why this is the case from Ketuboth 76. There is an exchange of an ass with a cow. The owner of the ass now get the cow. But when the owner of the cow goes to take possession of the ass he finds it dead. Rav Yehuda said in the name of  Shmuel the owner of the ass has to show it was alive at the time of the deal. Tosphot askes over there but why? The owner of the ass already has a hazaka (status) in the cow. [He has already taken possession,] So should not the other party have to bring the proof the ass was not alive at the time of the deal? Answer. It is only a Hazaka. That is not enough when we need clear proof, not just prior assumptions. So we see from this that Where you need proof a Hazaka (status) is not enough, but a hazaka (status) with a rov (majority) would be. [Why would the Rov help? I did not look into this enough yet. But I assume it comes from that Gemara in Ketuboth where you collect the Ketubah because she got married in a garment that is reserved for virgins. That is a Rov (majority).]