Internal contradiction in Mishna Bava Kama 3:11?

Question

The mishna in Bava Kama 3:11 brings a case:

היו שניים רודפין אחר אחד, זה אומר שורך הזיק, וזה אומר שורך הזיק--שניהם פטורין; ואם היו שניהם של איש אחד, חייבין

Two [oxen] were pursuing one [ox, and the pursued one was gored]; [the owner of] this one says "Your ox did the damage" and [the owner of] the other one says "Your ox did the damage" - both of them are exempt [from paying damages]; but if both belong to one person, he is liable.

The gemara explains that in the latter case, the owner only has to pay up to the value of the less valuable animal. So far so good. Ready for the next part of the mishna?

היה אחד גדול ואחד קטן, הניזק אומר גדול הזיק, והמזיק אומר לא כי, אלא קטן הזיק... המוציא מחברו, עליו הראיה

If one [ox] was big and the other small, and the plaintiff claims that the larger ox did the damage while the defendant claims that the smaller ox did the damage, the burden of proof rests on the plaintiff.

Understandable at face value, right? But then Kehati, paraphrasing the Bartenura, explains that this actually means that if the plaintiff cannot prove his claim, then the defendant doesn't even have to pay the lesser value!

Is this not a direct contradiction to the first part of the mishna, i.e. "if both belong to one person, he is liable"?

Answer 1

I don't have Kahati but from the Bartenura I will say that we are dealing with a legality in the second case that doesn't apply to the first. In the second case the plaintiff points to the big ox and says this one damaged mine. To which the defendant responds by pointing to a different, smaller ox and says no, this one did. Now, although value wise the smaller one's value is included in the larger one, the fact of the matter is the plaintiff is demanding a specific item, that is ox A, and the defendant is admitting a different item, ox B. This results in the defendant not having to pay, for he admit a different item than was demanded of him.

This does not apply to the first case where the defendant never pointed to a particular ox, he just says one of these damaged mine and being they both belong to one person, he is liable by default.